A body rub is almost the same as a massage in my mind, but with two distinct differences. A body rub consists of an amateur (vs. licensed/professional) massage of the body, done by a non-licensed person. Since it's not a professional arrangement and the purpose is more for touch, pleasure and feeling good, engaging in sexual activity during or at the end of the body rub is not at all out of the question.
For the most part, I've encountered basically two situations when giving/receiving a body rub. One is when the rub is given to one man from another who is non-licensed to perhaps earn some extra cash. By giving other men 'body rubs' he provides them a sensual touch experience, which usually is accompanied by a release at the end of the time.
The other circumstance is the one I'm more familiar with and it involves two men exchanging body rubs between each other. In this way to like-minded men who enjoy giving/receiving body rubs, exchange touch with each other for fun and pleasure. Exchanging body-rubs with other men is an incredible way to express healthy sexual maleness, explore your own body being touched in a safe and sensual way, and explore the differences in other male bodies all at the same time. Everyone needs to be touched to some degree or another and it can be a very affirming and comforting experience, in addition to the pleasurable aspects that present themselves when two men are naked and touching.
Along this line of thinking are also some guys who simply enjoy touching attractive or muscular men and who offer to give body rubs or massage in a one-way situation. The idea is the 'beautiful' man gets a free body rub and feels good, and the other man enjoys touching a naked man he finds attractive, which he probably, otherwise wouldn't have the opportunity to.