The scratch race is the most common style of cycling race, and it's also the same style race you probably did as a kid on your bike. You have a starting line and a finish line, both riders start in equal positions and ride to the finish. As simple as it sounds, there are some big strategies involved in being successful with this type of race.
Scratch races have varying lengths, and the longer the race, the more interesting it gets. If you've watched any sort of cycling racing, you've seen them riding in packs to reduce drag and save energy. This is a common strategy for riders that aren't as strong as the pack leaders. once they split from the pack (or peloton, in cycling jargon) they have fresh legs to sprint to the finish. Riding in the peloton can reduce drag on riders by as much as 40%, which makes it a very effective strategy!
The problem that you run into by riding with the pack is that if the leaders are sprinting ahead, you are stuck with the pack. This becomes very clear when the pack is lapped by those in the front. The velodrome track is fairly small, so it isn't tough to get lapped. In the scratch race, it is effectively over once the leader has completed because the ref has the power to "call" the race if the final results are clear.
Scratch racing is a full on sprint that is designed for the best athletes. It takes an even blend of speed and smarts to be able to make it to the front of the pack, while maintaining enough energy to sprint for the finish line. This blend of skills makes this the most common style of cycling racing, as well as one of the best to watch!