Muscle fibers can be classified in three categories: Slow twitch or type 1, type 2a and fast twitch or type 2b. Type 2a are essentially fast-twitch fibers but have a somewhat trainable ability to adapt the traits of both type 1 and 2b fibers. It's really dependent on the type and bulk of training you do. Type 2a are normally classified as intermediate fibers. The differences between the fiber types are based on chemical, functional and structural differences.
Slow-twitch fibers exhibit a very good blood supply and are sometimes referred to as red fibers. This large supply of blood ensures that slow-twitch fibers receive large amounts of oxygen, which provides them the capacity to perform for extended periods. They have very little potential for hypertrophy and are best suited to endurance activities such as running and cycling long distances. In contrast, fast twitch fibers have a relatively poor blood supply and are subsequently referred to as being white in color. This lack of blood results in relative oxygen restriction, so fast-twitch fibers tend to fatigue much faster than the better oxygenated slow-twitch fibers. Fast twitch fibers tend to be physically larger than slow-twitch fibers and also have the greatest hypertrophy or growth potential. These fibers are best suited to high-intensity but short-duration sports such as lifting heavy weights or sprinting.
We humans have a baseline ratio of Type I to Type II muscle fiber. This ratio is genetically determined. In most people, the ratio of slow- to fast-twitch muscle fiber is around 50/50. In elite endurance athletes, however, it can be skewed as much as 80/20 either way: world-class Olympic sprinters can exhibit 80% fast-twitch fiber in their legs, while the muscles of ultra-long distance runners can reach 80% slow-twitch fiber. If you are curious to determine your personal ratio there are methods to determine it including a needle biopsy where a small fragment of muscle is extracted and analyzed but some less intrusive methods are being developed.