A limiter is a compressor with the compression ratio set to 1:infinity.
The limiter/enhancers you can buy let you bring the ratio down a bit, but it's still very much a hard compression ratio. These pedals are really good if your dynamics are all over the place, or, if you want a very consistent level. The "enhance" circuit could help your passive sound a bit active, but that's really not the point of these pedals. The reason I like them is that you can set the threshold, and the ratio.
A compressor/sustainer will generally have less of a hard ratio. That's about all I can say (probably should have kept my mouth shut)
The most important point IMHO when deciding what kind of compressor to buy is that the limiter/enhancer pedals (boss+behringer) give you more control over the compression taking place.