As long as you edit source files inside a working directory you can always find out the state of your files via `cvs status' and `cvs log'. But as soon as you export the files from your development environment it becomes harder to identify which revisions they are.
CVS can use a mechanism known as keyword
substitution (or keyword expansion) to help
identifying the files. Embedded strings of the form
$keyword:...$ in a file are replaced
with strings of the form
$keyword:value$ whenever you obtain
a new revision of the file.