$ cvs tag -b rel-1-0-patches
This splits off a branch based on the current revisions in the working copy, assigning that branch the name rel-1-0-patches.
It is important to understand that branches get created in the repository, not in the working copy. Creating a branch based on current revisions, as the above example does, will not automatically switch the working copy to be on the new branch. For information on how to do that, see the section called “Accessing branches”.
You can also create a branch without reference to any working copy, by using rtag:
$ cvs rtag -b -r rel-1-0 rel-1-0-patches tc
-r rel-1-0 says that this branch should be rooted at the revision that corresponds to the tag rel-1-0. It need not be the most recent revision - it's often useful to split a branch off an old revision (for example, when fixing a bug in a past release otherwise known to be stable).
As with tag, the -b flag tells rtag to create a branch (rather than just a symbolic revision name). Note that the numeric revision number that matches rel-1-0 will probably be different from file to file.
So, the full effect of the command is to create a new branch - named rel-1-0-patches - in module tc, rooted in the revision tree at the point tagged by rel-1-0.