which/whereis differences

What's the difference between which and whereis ?

How about learning about whereis and which using whatis?

$  whatis which
which                (1)  - shows the full path of (shell) commands

$  whatis whereis
whereis              (1)  - locate the binary, source, and manual page files for a command

Basically, whereis searches for "possibly useful" files, while which only searches for executables.

I rarely use whereis. On the other hand, which is very useful, specially in scripts. which is the answer for the following question: Where does this command come from?

$  which ls
/bin/ls

$  whereis ls
ls: /bin/ls /usr/share/man/man1p/ls.1p.bz2 /usr/share/man/man1/ls.1.bz2

whereis searches the standard *nix locations for a specified command.

which searches your user-specific PATH (which may include some of the locations whereis searches, and may not include others - it might also include some places that whereis doesn't search if you'd added to your PATH)