Osteochondromas are the most common benign tumors of the bones.The tumors take the form of cartilage-capped bony projections or outgrowth on the surface of bones exostoses. It is characterized as a type of overgrowth that can occur in any bone where cartilage forms bone. Tumors most commonly affect long bones about the knee and in the forearm. Source: Wikipedia
It's estimated that they affect around 3% of the global population, usually between the ages of 10 and 30. It most often affects the the bones of the pelvis, leg, arms and shoulder blade.
The cause is generally unknown, though some hereditary factors are strongly suspected for some types.