Thermoset composite materials are sold by manufacturers in "grades" which denote the materials used (e.g., resin, matrix/laminate, additives) and often the intended application. The grades for these materials are assigned by NEMA and often MILSPECs.
G10 (also referred to as FR4) is defined as a "A continuous woven glass fabric laminated with an epoxy resin."
G11 is basically the same type of material, but is formulated "for continuous use at elevated temperature. G-11 retains at least 50% of its structural strength at continuous operating temperatures in excess of 300°F"
G9/G5 is a different beast - which is based on melamine resin and a woven glass fiber matrix. Melamine resin is typically much harder and more brittle than epoxy.
G7 is a high-temperature glass fiber laminate that uses a silicone resin.
GPO3 is a different type of materials - it uses random (not woven) glass fibers embedded in a polyester resin. It is less expensive, tough and not brittle.
There are many other "grades" of composites out there - including the phenolic grades, which we generally refer to as "Micarta". The material called G3 (also called PhenoGlas) is a phenolic resin that uses woven fiber matrix rather than the cotton cloth used in Micarta grades - basically a fiberglass Micarta.
TedP
P.S. - I just served as a counselor for the new Boy Scout Composite Materials Merit Badge. It was really cool teaching Scouts about the different properties and uses of composites!
