Man page
Regular usage is just man followed by the name of the command you want to know about.
man ls
Really Useful: You can use -t to format the output of man as paginated postscript and pipe the results to ps2pdf to make a PDF file!
man -t ls | ps2pdf - > ls.pdf
manpdf
Hard to remember the commands for making a PDF? So create a bash script that's in your path (for instance ~/bin/manpdf),
with:
#!/bin/bash
man -t $1 | ps2pdf - > $1.pdf
And be sure to chmod +x it so that you can then use it anytime you want to generate a PDF for intensive consulation.
For example:
manpdf mplayer
to create mplayer.pdf