This is Carfax. It has nothing to do with used cars, but is the site of the ancient crossroads in the Saxon town--where the east, west, north, and south roads crossed. Some think the name comes from "quatre voys" or "four ways." It is the heart of the commercial part of the City Centre.
Carfax Tower, all that remains of a 13th-century church. If you're physically fit you can climb up in the tower and get a good aerial view of Oxford. The clock has what are called "quarter boys" that strike the bell every 15 minutes. Or so I am told--I didn't hear them myself.