Concurrent Versions System (CVS) Question:

CVSROOT environment variable, setenv CSVROOT (csh) or export CSVROOT= (bash)

If you have changed a file in CVS and want to commit your changes to the repository, type:
cvs commit filename

Checkout package or module:
cvs checkout module

To add a new file to the repository, first create the file in your checked out copy and then type:
cvs add -m'Brief description.' filename

Deleting a file involves similar steps. First delete the file out of your checked out copy, then use cvs remove to mark it as deleted, and then use cvs commit to commit the change:
rm filename
cvs remove filename
cvs commit filename

You can combine the first two steps by using:
cvs remove -f filename

If you want to get a report on what's changed between your copy and the repository, but don't want CVS do actually do anything, use the command:
cvs -n update

To see the differences (in diff format) between a file you've modified and the last committed revision in the repository, use:
cvs diff filename

(You can also give cvs diff multiple filenames or even whole directories and it will produce a series of diffs.) cvs diff takes the standard flags to specify what type of diff to produce, so:
cvs diff -u filename
will produce a unified context diff (the type that most people prefer).