Man is probably the most used unix/linux command in history. Man formats and displays the on-line manual pages. You could think of man as the dictionary of unix commands. When you need to know what a word means, you look in a dictionary, find the word, and find its meaning along with its pronunciation and how it is used. Man is very similar, except it deals with information related to commands. You will find that you often need to refer to the man page to find out about a program, find out about its options, etc.

From the linux man man page (nope, that was intended, to access the man page for man, you use the command man man):

Sometimes you will see a command and don't know for sure what it does. An easy way to tell is to use the whatis command to find the relevant information.

First, whatis is described in the man page as:

Have you ever wanted to accomplish something in linux but wasn't sure how to do it? Or maybe, you know there is a command that does something but can't remember the command? We have all run into this. With linux, there are literally thousands of commands and we can't possibly remember them all. To help with this situation, we have a command known as apropos.

From the apropos man page: