Micarta is an industrial laminate, originally used in electrical and decorative applications, produced by applying heat and pressure to layers of fibrous materials -- such as linen, burlap, canvas or paper -- which are impregnated with epoxy or similar resin.
G-10, sometimes called Garolite, is similar to Micarta, except that fiberglass is used as the substrate (instead of canvas, linen, etc.).
Both materials are extraordinarily tough, resist absorbing moisture and are available in various colors and finishes (smooth, matte and rough). Generally, G-10 is slightly lighter.
In the laboratory, G-10 edges out Micarta for strength. For knife handles, however, there's no practical difference between the two. Both are nearly indestructible, so buy with confidence.