There is much documentation to be found on most Unix-like systems, such as Linux or Mac OS X. Most commands have a manual page, which you can get from the terminal prompt.
Lets say we want to know about a command called ls, which is a command for listing files (much like the dir command under DOS). I should point out that there are GUI tools for this, but where we're going in this course, there is no GUI. Manual pages are sorted under a number of sections, as described under the manual page for man manual page. If you see a notation like man(1), then that means to look under section 1. Most of the time you don't need to specify a section, it will take the first match it finds.
man 1 lsThe section is optional
You can use the arrow keys, space, page up/down to navigate. When you're done, type q to exit. Notice how each manual page describes a brief synopsis about what the command does, its options with their meanings, various notes and other bits and pieces.
Have a look at the man(1) manual page and have a quick look at what sections there are. Most of what you'll need will be found in sections 1, 5 and 8.