More than 100 standard UNIX commands and utilities exist. Each one may have options; some commands may have 15 options or more. These options are not universal, so how on earth are you supposed to remember all the options for all the commands? Luckily, you don’t have to. UNIX maintains a set of online manuals with information about each command and its options.
You use the man command to access this information:
$ man ls
This command shows you the online manual pages that describe how to use the ls command. This process is called checking the man page on ls. Most commercial UNIX systems have similar man pages. As an example, take a look at the man page for ls on my Solaris system: