ectomorph=Naturally skinny individuals with fast metabolisms.They seem to stay very lean but have trouble gaining muscle and need to eat quite a bit to gain.Usually lightly built with a small bone structure.
mesomorph=naturally muscular individuals with moderate metabolisms.Can gain muscle pretty fast and gain fat at a moderate pace.Usually medium/heavy build with fairly thick bone stucture.Most pro bodybuilders would be in this category.
Endomorph=Naturally chubby individuals with a slow metabolisms.Can gain muscle as fast/if not faster then mesomorphs,but also gains fat alot of fat with it.Most have very wide hips and narrow shoulders.Most superheavyweight powerlifters would be a good example of this physique
Most people will be a combination of 2types,I myself am a endo/meso tending towards endomorph.
Sorry if it is somewhat vague,but thats all I could think of at the time.
hope it helps