In CVS, revision numbers are per-file. This is because CVS stores its data in RCS files; each file has a corresponding RCS file in the repository, and the repository is roughly laid out according to the structure of your project tree.

In Subversion, the repository looks like a single filesystem. Each commit results in an entirely new filesystem tree; in essence, the repository is an array of trees. Each of these trees is labeled with a single revision number. When someone talks about “revision 54”, they're talking about a particular tree (and indirectly, the way the filesystem looked after the 54th commit).

Technically, it's not valid to talk about “revision 5 of foo.c”. Instead, one would say “foo.c as it appears in revision 5”. Also, be careful when making assumptions about the evolution of a file. In CVS, revisions 5 and 6 of foo.c are always different. In Subversion, it's most likely that foo.c did not change between revisions 5 and 6.

For more details on this topic, see the section called “Revisions”.