I took my bike to the LBS for tune up/inspection. The guy said my chain had stretched (apparently this is pretty common) and that it needs to be replaced at some point soon. He also said that when you replace the chain you generally want to replace the rear cassette. (something about the chain being fit to it, I don't really remember the reason)

So my question is: is it generally adviseable to replace the cassette with the chain? And should I just get the same cassette? I have the original 9 speed that came with the Trek 1.2. I've searched the forum, and have found some info on cassettes but alot of it is over my head. I don't know what an 11-28 cassette is or means. Anyway I'm going to take my bike up there sometime this week, so I need to make a decision.

Thanks in advance for the help!