Epoxy resin. They can either use the polyester resin when they're laminating your board or they can use epoxy resin. Okay epoxy resin is more expensive. Your board's going to usually cost $100 to $150 more, but the epoxy resin is much stronger. Okay, they can use it on a regular polyester blank, and the board will be really strong. But usually they use the epoxy resin on an EPS Styrofoam blank. Now the Styrofoam blank is a much lighter, much weaker blank, and basically, they use the epoxy resins with the fiberglass to make the outer shell of that as strong as possible.
Now with epoxy's, because you can use the EPS Styrofoam, the boards become really light. You can have a big, longboard and have it cut down half the weight on it by using epoxy and EPS Styrofoam. Most boards still are made with a polyester resin, but there are some boards now that are made with epoxy that are very strong.
Now some surfers like the stiffer feel of the epoxy's, but most surfers like the feel and the flex of your polyurethane blank and your polyester resins. But still, if you're going to buy a board, and you want it for a really long time, epoxy resins are probably the way to go.