> man <command name>

For example, to find out about the man command itself, you would enter:

> man man

If you don't know the name of the command, but only the general topic, you can use the apropos command,

> apropos <topic>

which lists all the man pages related to the specified topic.

Man pages are stored as nroff files.

Yahoo!'s Man Pages page
Yahoo!'s directory of Man Pages.