What are leaf springs? Why buy leaf springs?
Leaf springs consist of several layers of metal "leaves" which are bound together to act as a single unit. They the primary suspension component used for the rear of most trucks, and are built to support heavy loads. Buying new lowering leaf springs allows you to lower the rear of your truck, since they are built with a different spring rate or leaf setup than factory springs. There are a number of ways to lower the rear of your truck, and buying new leaf springs one of the popular ways to do this. For an extreme drop of your truck, many people buy new leaf springs and a block kit, both of which add to the total amount of drop.

Can any vehicle use leaf springs?
Only vehicles that utilize factory leaf springs would be able to incorporate replacement aftermarket leaf springs.

How do you install leaf springs?
Your factory leaf pack must be unbolted from the front and rear perches. Then simply install the new leaf springs.