The 88 is a Mossberg Maverick model. It's the more utilitarian version of the two. The barrels are interchangeable between the 88 and 500. The only real difference is the safety location. On the 500 its on top, on the 88 its on the trigger guard. The forends can be different also.
Mossberg 500 is what I started duck hunting with. It was my first shotgun. Great gun! Don't know the difference, but know the 500 will last you forever.