Tags for Emacs: Relationship between etags, ebrowse, cscope, GNU Global and exuberant ctags

That's as a good question as I've recently read here, so I'll try explain the difference in more detail:

Point 1:

etags and ctags both generate an index (a.k.a. tag/TAGS) file of language objects found in source files that allows these items to be quickly and easily located by a text editor or other utility. A tag signifies a language object for which an index entry is available (or, alternatively, the index entry created for that object). The tags generated by ctags are richer in terms of metadata, but Emacs cannot interpret the additional data anyways, so you should consider them more or less the same (the main advantage of ctags would be its support for more languages). The primary use for the tags files is looking up class/method/function/constant/etc declaration/definitions.

cscope is much more powerful beast (at least as far as C/C++ and Java are concerned). While it operates on more or less the same principle (generating a file of useful metadata) it allows you do some fancier things like find all references to a symbol, see where a function is being invoked, etc (you can find definitions as well).

To sum it up:

ctags one allows you to navigate to symbol declaration/definitions (what some would call a one-way lookup). ctags is a general purpose tool useful for many languages.

On the other hand (as mentioned on the project's page) cscope allows you to:

Go to the declaration of a symbol
Show a selectable list of all references to a symbol
Search for any global definition
Functions called by a function
Functions calling a function
Search for a text string
Search for a regular expression pattern
Find a file
Find all files including a file

It should come as no surprise to anyone at this point, that when I deal with C/C++ projects I make heavy use of cscope and care very little about ctags. When dealing with other languages the situation would obviously be reversed.

Point 2.

To have intelligent autocompletion you need a true source code parser (like semantic), otherwise you won't know the types of the objects (for instance) in your applications and the methods that can be invoked on them. You can have an autocompletion based on many different sources, but to get the best results you'll ultimately need a parser. Same goes for syntax highlighting - currently syntax highlighting in Emacs major modes is based simply on regular expressions and that's very fragile and error prone. Hopefully with the inclusion of semantic in Emacs 23.2 (it used to be an external package before that) we'll start seeing more uses for it (like using it to analyse a buffer source code to properly highlight it)

Since Emacs 24.1 semantic is usable from the Emacs completion framework. The easiest way to test it is to open up a C source code file and typing M-TAB or C-M-i and watch as semantic automagically completes for you. For languages where semantic is not enabled by default, you can add it the following line to your major mode hook of choice:

(add-to-list 'completion-at-point-functions 'semantic-completion-at-point-function)

Point 3.

semantic brings true code awareness (for the few languages it currently supports) and closes the gap between IDEs and Emacs. It doesn't really interface with tools like etags and cscope, but it doesn't mean you cannot use them together.

Hopefully my explanations make sense and will be useful to you.

P.S. I'm not quite familiar with global and ebrowse, but if memory serves me they made use of etags.

I'll try to add some explanations to 1.

What is it?

Etags is a command to generate 'TAGS' file which is the tag file for Emacs. You can use the file with etags.el which is part of emacs package.
Ctags is a command to generate 'tags' file which is the tag file for vi. Universal Ctags, the successor of Exuberant Ctags, can generate 'TAGS' file by the -e option, supporting more than 41 programming languages.
Cscope is an all-in-one source code browsing tool for C language. It has own fine CUI (character user interface) and tag databases (cscope.in.out, cscope.out, cscope.po.out). You can use cscope from Emacs using xcscope.el which is part of cscope package.
GNU GLOBAL is a source code tagging system. Though it is similar to above tools, it differs from them at the point of that it is dependent from any editor, and it has no user interface except for command line. Gtags is a command to generate tag files for GLOBAL (GTAGS, GRTAGS, GPATH). You can use GLOBAL from emacs using gtags.el which is part of GLOBAL package. In addition to this, there are many elisp libraries for it (xgtags.el, ggtags.el, anything-gtags.el, helm-gtags.el, etc).

Comparison

Ctags and etags treat only definitions. Cscope and GNU GLOBAL treat not only definitions but also references.
Ctags and etags use a flat text tag file. Cscope and GNU GLOBAL use key-value tag databases.
Cscope and GNU GLOBAL have a grep like search engine and incremental updating facility of tag files.

Combination

You can combine Universal Ctags's rich language support and GNU GLOBAL's database facility by using ctags as a plug-in parser of GLOBAL.

Try the following: (requires GLOBAL-6.5.3+ and Universal Ctags respectively)

Building GNU GLOBAL:

$ ./configure --with-universal-ctags=/usr/local/bin/ctags
$ sudo make install

Usage:

$ export GTAGSCONF=/usr/local/share/gtags/gtags.conf
$ export GTAGSLABEL=new-ctags
$ gtags                     # invokes Universal Ctags internally
$ emacs -f gtags-mode       # load gtags.el

(However, you cannot treat references by this method, because ctags don't treat references.)

You can also use cscope as a client of GNU GLOBAL. GLOBAL package includes a command named 'gtags-cscope' which is a port of cscope, that is, it is cscope itself except that it use GLOBAL as a search engine instead of cscope's one.

$ gtags-cscope          # this is GLOBAL version of cscope

With the combinations, you can use cscope for 41 languages.

Good luck!

TAGS files contain definitions

A TAGS file contains a list of where functions and classes are defined. It is usually placed in the root of a project and looks like this:

^L
configure,3945
as_fn_success () { as_fn_return 0; }^?as_fn_success^A180,5465
as_fn_failure () { as_fn_return 1; }^?as_fn_failure^A181,5502
as_fn_ret_success () { return 0; }^?as_fn_ret_success^A182,5539
as_fn_ret_failure () { return 1; }^?as_fn_ret_failure^A183,5574

This enables Emacs to find definitions. Basic navigation is built-in with find-tag, but etags-select provides a nicer UI when there are multiple matches.

You can also uses TAGS files for code completion. For example, company's etags backend uses TAGS files.

TAGS files can be built by different tools

ctags (formerly known as 'universal ctags' or 'exuberant ctags') can generate TAGS files and supports the widest range of languages. It is actively maintained on github.

Emacs ships with two programs that generate TAGS files, called etags and ctags. Emacs' ctags is just etags with the same CLI interface as universal ctags. To avoid confusion, many distros rename these programs (e.g. ctags.emacs24 on Debian).

There are also language specific tools for generating TAGS files, such as jsctags and hasktags.

Other file formats

ebrowse is a C program shipped with Emacs. It indexes C/C++ code and generates a BROWSE file. ebrowse.el provides the usual find definition and completion. You can also open the BROWSE file directly in Emacs to get an overview of the classes/function defined a codebase.

GNU Global has its own database format, which consists of a GTAGS, GRTAGS and GPATH file. You can generate these files with the gtags command, which parses C/C++ code. For other languages, GNU Global can read files generated by universal ctags.

GNU Global also provides a CLI interface for asking more sophisticated questions, like 'where is this symbol mentioned?'. It ships with an Emacs package gtags.el, but ggtags.el is also popular for accessing GNU Global databases.

Cscope is similar in spirit to GNU Global: it parses C/C++ into its own database format. It can also answer questions like 'find all callers/callees of this funciton'.

See also this HN discussion comparing global and cscope.

Client/Server projects

rtags parses and indexes C/C++ using a persistent server. It uses the clang parser, so it handles C++ really well. It ships with an Emacs package to query the server.

google-gtags was a project where a large TAGS file would be stored on a server. When you queried the server, it would provide a subset of the TAGS file that was relevant to your search.

Semantic (CEDET)

Semantic is a built-in Emacs package that contains a parser for C/C++, so it can find definitions too. It can also import data from TAGS files, csope databases, and other sources. CEDET also includes IDE style functionality that uses this data, e.g. generating UML diagrams of class hierarchies.

[answer updated from shigio's]

I'll try to add some explanations to part 1 of the question.

What is it?

Etags generates a TAGS file which is the tag file format for Emacs. You can use an Etags file with etags.el which is part of Emacs.
Ctags is the generic term for anything that can generate a tags file, which is the native tag file format for Vi. Universal Ctags (aka UCtags, formerly Exuberant Ctags) can also generate Etags with the -e option.
Cscope is an all-in-one source code browsing tool for C (with lesser support for C++ and Java), with its own tag databases (cscope.in.out, cscope.out, cscope.po.out) and TUI. Cscope support is built-in to Vim; you can use Cscope from Emacs using the xcscope.el package. There are also Cscope-based GUIs.
GNU GLOBAL (aka Gtags) is yet another source code tagging system (with significant differences--see next section), in that it also generates tag files.

Comparison

Ctags and Etags treat only definitions (of, e.g., variables and functions). Cscope and Gtags also treat references.
Ctags and Etags tag files are flat. Cscope and Gtags tagfiles are more powerful key-value databases, which allows (e.g.) incremental update.
Cscope and Gtags have a grep-like search engine.
Ctags has built-in support for more languages and data formats: see the current-in-repository list of Universal Ctags parsers. UCtags also has documented how to develop your own parser.
Cscope and Gtags are editor-independent.
Gtags does not provide its own user interface, but can currently (Oct 2016) be used from commandline (CLI), Emacs and relatives, Vi and relatives, less (pager), Doxygen, and any web browser.
Gtags provides gtags.el via the GLOBAL package, but there are also many other elisp extensions, including xgtags.el, ggtags.el, anything-gtags.el, helm-gtags.el.

Combination

You can combine Universal Ctags' rich language support with Gtags' database facility and numerous extensions by using Ctags as a GLOBAL plug-in parser:

# build GNU GLOBAL
./configure --with-exuberant-ctags=/usr/local/bin/ctags
sudo make install

# use it
export GTAGSCONF=/usr/local/share/gtags/gtags.conf
export GTAGSLABEL=ctags
gtags                     # invokes Universal Ctags internally
emacs -f gtags-mode       # load gtags.el

Note again that if you use Ctags as the parser for your Gtags, you lose the ability to treat references (e.g., variable usage, function calls) which Gtags would otherwise provide. Essentially, you trade off Gtags' reference tracking for Ctags' greater built-in language support.

You can also use Cscope as a client of Gtags: gtags-cscope.