Fast and slow twitch are generalizations for more specific muscle fiber types. Physiologists classify human muscle fibers as type 1( slow twitch )type 2 (fast twitch) In general, slow twitch muscle fibers are relatively fatigue resistant, but do not contract as rapidly or as strongly.
Fast-twitch fibers contract more rapidly and forcefully, but fatigue more quickly. Although all muscle fiber types respond to INTENSE training, fast twitch fibers tend to have greater ability for hypertrophy (growth of lean muscle tissue).
What does all this mean? Training with a program such as the one a preach about on this site (H.I.G.T) high intensity group training promotes the recruitment of fast twitch muscle fibers. A result driven workout that enables muscles to produce more power, boosts metabolism, raises the metabolic rate to enhance calorie burning long after you leave the gym.

Richard Seymour