There is a fairly recent thread here that asks a very similar question, check out:
http://www.disboards.com/threads/vistana-vs-vistana-villages.3556655/
Perhaps the easiest way to explain it is to explain a little history. Sheraton Vistana Resort (SVR) started out in early 70's as a tennis training resort rather than what we now think of as timeshares. Over the years it grew considerably and expanded out to 7 different sections over a fairly large chunk of property at I-4 and 535. Over those years the resort morphed a bit as the timeshare industry evolved. The first section, Courts, has more what you would think of as stacked townhomes with lots of open space. As the resort expanded, the building density and height went up, so that the last two sections (Lakes and Cascades) were 5-6 story buildings with elevators and outside corridors.
I'm not certain exactly when Vistana Villages (SVV) started construction, but presumably it was as SVR was running out of space and the timeshare boom was still in full flight. Although it wasn't that serial, as both were under active development at the same time for a period. SVV is about 4 miles away from SVR, a bit closer to SeaWorld a little bit further from Disney (but still very handy access to all the parks). The resort is a bit smaller, and the buildings are all similar to the newer two sections of SVR. SVR is bigger, has a few more amenities, and has more variety of accommodations to suit different tastes. Our very first timeshare stay was at SVV many years ago, where we caught the bug, and we later purchased a week at SVR (resale!) to feed our Orlando habit.
Either resort is a great choice, and they are both well kept to Sheraton standards (although Sheraton has since spun off it's timeshare division). SVR had a multi-million dollar refurbishment just a few years ago so even the oldest buildings are decked out with granite countertops, heavenly beds, and other top finishes. The larger SVR resort has a fairly strong following here on DIS, join us on the SVR thread here:
http://www.disboards.com/threads/the-sheraton-vistana-resort-thread.3148502/ for pictures and details on the resort. Feel free to ask any questions.