Despite the fact that the term resin is used here a lot, "resin" is a generic term used for all liquid polymers. Typically, fiberglass is strengthened with polyester, or epoxy resin. Polyester is cheaper and more common, but smells. Epoxy is stronger, more expensive, more difficult to work with, but has less odor.
Typically, the resin used in casting is urethane resin. You can use it with fiberglass, but that's a whole other ball of wax.
What do you want to make?.