Hi Scotty just copied this off Google Answers hope it helps:question:
The most obvious visible difference is a baitcasting reel has a level-wind mechanism, while a "trolling" (that is, conventional) reel doesn't. But beyond that, a baitcaster is made to be used on a baitcasting rod (generally with a trigger grip), and during the retrieve, held with the left hand below or cupping the reel, while a conventional reel is made to be held with the left hand above the reel (and the butt of the rod against the fisherman's waist), allowing more leverage when pulling on a bigger fish with heavier line.
But then, there are conventional reels made for casting, and conventional reels made for trolling. They look pretty much the same. The ones made for casting have lightweight spools (aluminum or graphite, or in the old days, plastic), and with a bit of practice you can cast one a long ways. The ones made for trolling often have heavy plated brass spools. These heavy spools make casting next to impossible. But many so-called trolling reels (the big Penn Senators and Internationals, for example) are now made with aluminum spools, and people do cast them pretty good distances -- of course, you use a heavy bait or lure, but then you don't use these for little fish.
And why would you cast with a big trolling reel? To catch a big fish. You can't land 100-pound tuna on a baitcasting reel full of 17 pound line, at least not with any