A body lift raises the actual body of the truck off of the frame. Body lifts are generally cheaper in price, but a little more difficult to install.
A suspension lift raises the entire vehicle..frame included, by means of installing larger springs/coils, or in the case of the Xterra, longer upper control arms up front. For the rear of the Xterra, there are a few different ways to lift it. You have AAL's (Add A Leaf) Longer rear shackles, or new leaf springs.
Here's a short article on the subject from offroad.com
suspension lift vs body lift