Differences between standardbreds and thoroughbreds?

I have a standardbred myself. In my barn, there are a couple other standardbreds and thoroughbreds. I realize the differences in racing techniques, such as pacing, carts,galloping, trotting, etc. What i would like to know, is what is the bloodline difference?? Are standardbreds just thoroughbreds with a domestic selection of alleles? Or are they a complete different breed?