Ok,
lets dig deeper since google isnt getting used, and there is a WHOLE LOT of information to be read and learned about composites on the interweb

you have:

Polyester resin: this is a polyester resin that is typically catalized using MEK as a hardener. The "Bondo" brand fiberglass kits you see at walmart and auto parts store, that is polyester resin.

Then you have:

epoxy resin: This also is a two part chemical. Depending on its intended use and manufacturer, its consistency could be anywhere from water thin, to "thixotropic" which means it doesnt run or sag when placed on a vertical surface.
Its mixing ratio's also vary greatly.

The epoxy you see in a hobby shop is what i would call a syrup consistency and is used to bond parts together when a glue joint needs to be stronger than what a CA adhesive can provide, and are typically mixed in equal parts A and B.

The problem with the thicker consistency epoxies at the hobby store is they have short work times and are very difficult if not impossible to use in "wetting out" fiberglass cloth.

That is where "laminating epoxies" come into place. Brand names include: West systems, adtech, resin research, and many more. These epoxies are typically Not mixed in equal parts. They can also have different types of hardeners to change the working time and the physical cured properties of the cured resin. These resins are also much thinner in consistency. This thinness of consistency allows the resin to work into and "wet out" the fiberglass cloth more quickly and with more ease. These resins are typically not used as structural "bonding" resins due to them being thin and running away from the glue joint. To use them as a structural bonding resin, filler materials must be used to change the properties of the resin.

When it comes to fiberglassing something like a balsa structure, making a fiberglass mold, or laying up a fiberglass fuselage/cowl/etc, a "laminating resin" is used. When you want to bond parts to a fiberglass molded parts, you want to use a structural resin, such as Hysol or a quality hobby grade epoxy resin.