Here's a link to Mossberg's site:
http://www.mossberg.com/
All of these Mossberg shotguns are based on the same basic cast aluminum receiver design.
Most receiver and trigger parts are interchangeable, and except for the 590-A1, all use plastic trigger groups and safety switches.
The 500 is their "standard" gun.
It's chambered for the 3" Magnum shells and is available in a wide variety of sporting and defense configurations.
The 505 is a youth version of the 500. It's available only in 20 gauge and .410. It's got a short stock and shorter barrels to fit children and smaller people.
The 535 is Mossberg's "Super Magnum" model, chambered for the 3 1/2" Magnum shells.
The 590 series is Mossberg's premium Police and Military gun.
These are beefed-up guns with heavier barrels, and a barrel mounting system that's like Remington's 870 shotgun.
The 590-A1 is an even heavier-duty version with a metal trigger group, metal safety switch, and other features to make the gun strong enough for use by police agencies and for government sales to the military.