Like it says on the poster: something like an alleycat, except we don't really have "alleys" per se in Santa Cruz (population 55,000), so it's more like a mountain bike race on beach sand trails. With fixed gear bikes. It's kind of awesome. Geared commuter bikes complete with racks and panniers are also popular. I plan to be there so look for me and my camera.
The Santa Cruz Bike Church is on south Pacific Avenue -- the address is 703 Pacific but you actually have to turn on to Spruce to get to it.