The man command provides online displays of reference pages.
To use the command type:
% man man
% man -k keyword ...
The option -k and a set of keywords, searches through all the sections of the manual and looks for the word you specify to appear in the description of the command. If it does appear, then man -k will tell you what entries to look under using the man command.
For example, if you didn't know the cp command meant copy file but you knew there was some command for copying files, you could type man -k copy and the program would tell you to look at the manual page for cp. It will also give a long list of other commands that have the word copy in their descriptions.
% man -k copy
bcopy, bcmp, bzero, ffs (3) - bit and byte string operations cp (1) - copy cpio (1) - copy file archives in and out dd (1) - convert and copy a file rcp (1c) - remote file copy
The alphanumerals in parentheses indicate the volume of the manual where the respective description may be found. If you know which volume it's in you can specify the number in the man command. So we could type either man cp or man 1 cp.