Base jumping is not part of the skydiving industry. However, the majority of base jumpers started out skydiving before moving onto base jumping, so there is a strong relationship with skydiving.
Base jumping involves using a parachute after jumping from a fixed object: a building, pylon, bridge or cliff etc.
During the 80's and 90's it was common to see base jumpers using skydiving equipment. This is now frowned on, because experience has shown that base jumpers need specially designed equipment to improve safety.
Base jumping is significantly more dangerous than skydiving for many reasons. The most obvious is the low altitudes, resulting in short delays. A jump from 500 ft gives a base jumper about 5 seconds before he impacts the ground, if he fails to deploy his parachute correctly.
Learning to base jump is not as regulated as skydiving, which is why most base accidents happen. However, it is possible to find well qualified and professional base instructors, who also provide professional base jumping equipment.
Base jumping can be done legally but the majority of base jumps are illegal, because it's difficult to get property owners to give permission for a base jump.
Base jumping website's are a great source of information, so anyone thinking about this extreme sport can now do a lot of research first.