There are on-line manuals which gives information about most commands. The manual pages tell you which options a particular command can take, and how each option modifies the behaviour of the command. Type man command to read the manual page for a particular command.
For example, to find out more about the wc (word count) command, type
man wc
Alternatively
whatis wc
gives a one-line description of the command, but omits any information about options etc.
For some commands you can simply type
help cd
this command display helpful information about builtin commands.
When you are not sure of the exact name of a command,
apropos keyword
will give you the commands with keyword in their manual page header. For example, try typing
apropos copy