Suspension lift lifts the truck via raising up the suspension. A body lift raises only the body via spacers installed between the frame and the body. A suspension lift is typically 6" tall while the body lift is usually about 3".
I don't recommend a body lift at all - the suspension lift is more expensive and a better solution but many go with body lifts to save money. There are issues with the body lift - it will raise the body up above other suspension-mounted components which can be an esthetic problem. For example, the tow hitch typically will be 3" below the body.