DT lenses are digital lenses. These lenses offer ADI metering, different coating (helps to reduce flair, ghosting, etc.), and supposedly works better with SSS. The 70-210 (beercan) does not have the newer coating and utilizes TTL metering.

I've never had a flair or ghosting problem with older non-DT lenses. I do not doubt the other poster finds the 18-200 a good solution for his/her needs, different strokes...

Personally, I use the Sony 75-300 unless shooting portrait/candid, concerts (lowlight), or sports. In those cases I use the "beercan", as it is fairly sharp wide open and gives a slight speed advantage over most consumer zooms.