.o files vs .a files

What is the difference between these two file types. I see that my C++ app links against both types during the construction of the executable.

How to build .a files? links, references, and especially examples, are highly appreciated.

.o files are objects. They are the output of the compiler and input to the linker/librarian.

.a files are archives. They are groups of objects or static libraries and are also input into the linker.

Additional Content

I didn't notice the "examples" part of your question. Generally you will be using a makefile to generate static libraries.

AR = ar 
CC = gcc

objects := hello.o world.o

libby.a: $(objects)
    $(AR) rcu $@ $(objects)

%.o: %.c
    $(CC) $(CFLAGS) -c $< -o $@

This will compile hello.c and world.c into objects and then archive them into library. Depending on the platform, you may also need to run a utility called ranlib to generate the table of contents on the archive.

An interesting side note: .a files are technically archive files and not libraries. They are analogous to zip files without compression though they use a much older file format. The table of contents generated by utilities like ranlib is what makes an archive a library. Java archive files (.jar) are similar in that they are zip files that have some special directory structures created by the Java archiver.

A .o file is the result of compiling a single compilation unit (essentially a source-code file, with associated header files) while a .a file is one or more .o files packaged up as a library.