In linux there is a system called man (short for "manual"). For instance, say I forget the syntax of the printf function, typing "man printf" on the command line will open up the man page on printf. Inside the man page it will tell you what include files you need, what parameters the function takes, what the return values are, and sometimes most importantly, what similar functions exist.
Some people hate the using man for some reason, and would rather use a web site. Linux Questions helps out with the
Linux Man Pages section which allows you to search through them with your web browser.
