The man command displays reference pages for the command you spesify. For example;
There's about one full page of information about cat. Try it. Don't expect to understand it, though. It assumes quite some Unix\ knowledge. When you've read the page, there's probably a little black block at the bottom of your screen, reading -more-, Line 1 or something similar. This is the more-prompt, and you'll learn to love it.
Instead of just letting the text scroll away, man stops at the end of each page, waiting for you to decide what to do now. If you just want to go on, press and you'll advance a page. If you want to exit (quit) the manual page you are reading, just press . You'll be back at the shell prompt, and it'll be waiting for you to enter a new command.
There's also a keyword function in man. For example, say you're interested in any commands that deal with Postscript, the printer control language from Adobe. Type man -k ps or man -k Postscript, you'll get a listing of all commands, system calls, and other documented parts of Unix that have the word ``ps'' (or ``Postscript'') in their name or short description. This can be very useful when you're looking for a tool to do something, but you don't know it's name--or if it even exists!