man manIf you don't know exactly what you're looking for, you may want to try man -k keyword, where keyword is a word that might be used to describe the command you are looking for. This will give you a list of manpages that may be relevant.
Sun provides a documentation viewer called answerbook. If you are on a Sun system and you are using X windows, you can just type answerbook at the prompt.
GNU software, such as emacs, gcc, gdb, etc., is documented in GNU Info pages. To read info pages, start emacs, type C-h i, and follow the directions at the top of the screen.