Best way to demonstrate the difference is with an example.
If I tell you that you can't talk about drug use at all, that's a content-based restriction.
But if I tell you that you can't talk about how drug use is GOOD, that's a viewpoint restriction.
Essentially, the content is the subject matter, the viewpoint is the position the speaker takes on the subject matter.
In practice, however, the line is definitely blurred, and even though viewpoint restrictions are categorically unconstitutional, I think courts allow a lot through and call it "content restrictions."
(this is at least how I understand it, and I'm by no means a con law specialist!)