There are in general terms two distinct types of Resin's, Polyester and Epoxy, both have different properties, the Polyester resins use an exciter catalyst that pull the polymer chain together and move on to the next hair in the chain, but only if they are not caught in a area that has already set, therefore they shrink, Epoxy resins use a catalyst that binds to the hairs in the chain and therefore do not move on to the next chain, therefore less shrinkage and the requirement to mix properly to ensure all of the chains in the resin have sufficient catalyst touching the whole of the polyester chain, there are lots of other types of resins, extended with fillers etc, but as I said in general the two types are as mentioned, hope that helps.
Mike