Before we get too far in, we should warn you that there's going to be a lot of discussion of “changes” in the pages ahead. A lot of people experienced with version control systems use the terms “change” and “changeset” interchangeably, and we should clarify what Subversion understands as a changeset.
Everyone seems to have a slightly different definition of “changeset,” or at least a different expectation of what it means for a version control system to have one. For our purpose, let's say that a changeset is just a collection of changes with a unique name. The changes might include textual edits to file contents, modifications to tree structure, or tweaks to metadata. In more common speak, a changeset is just a patch with a name you can refer to.
In Subversion, a global revision number N names a tree in the repository: it's the way the repository looked after the Nth commit. It's also the name of an implicit changeset: if you compare tree N with tree N-1, you can derive the exact patch that was committed. For this reason, it's easy to think of revision N as not just a tree, but a changeset as well. If you use an issue tracker to manage bugs, you can use the revision numbers to refer to particular patches that fix bugs—for example, “this issue was fixed by revision 9238.” Somebody can then run svn log -r 9238 to read about the exact changeset that fixed the bug, and run svn diff -c 9238 to see the patch itself. And (as you'll see shortly) Subversion's merge command is able to use revision numbers. You can merge specific changesets from one branch to another by naming them in the merge arguments: svn merge -c 9238 would merge changeset #9238 into your working copy.