Micarta and G-10 (or Garolite) both are industrial laminates -- tough and moisture-resistant, available in a variety of colors and finishes. They're produced by applying pressure and heat to layers of fibrous materials (called "substrates") impregnated with resin.
The physical difference between the two materials is the substrates used to make them. Micarta employs canvas, linen, paper or burlap, while G-10 uses fiberglass.
In general, G-10 is a bit lighter and (in laboratory testing) ever-so-slighly stronger. For all practical purposes -- in knife handles, that is -- there's no difference. Both are pretty much indestructible, so choose the material that appeals to you.