Here's a short little essay I wrote for my English Composition class. I tried to describe the differences as simply as I could.

When playing Massive Assault Network (MAN), one of the most commonly asked questions is “What is the difference between this game and Massive Assault (MA)?” First off, let me describe MA. It is one of the best war games ever made, with a variety of maps to battle on with randomized starting locations, so that even on the same map, no two battles are the same. MA can be purchased at stores or ordered from an online store, while MAN must be downloaded from the company’s website. MAN has a monthly subscription fee, but for that price you get free registration for any tournaments, the ability to play in team competitions, free teaching games with the Mentors, a chat room, and a new map every month. You only pay once for MA, and it contains a bare-bones multiplayer with no chat and no new maps. MA is focused on single player game play, with epic battle scenarios, whereas MAN is all about multiplayer, with only a small amount of single player training missions. The fun comes from the human competition. Playing a brain-dead computer opponent is no fun at all (for me anyway). MAN also has great support. There is a great deal of interaction between the community that has sprung up around this game and the developers, so that problems are resolved quickly. MAN is being constantly updated and improved. Some of the features that are now available in the game came about simply because the people who play asked for them. Actually, we nagged the developers a lot, but you get the idea. MA also has great support, but it is updated less often than MAN, the main reason being that MA was released a year earlier than MAN was, so it is much more stable. MA/MAN has much to offer any fan of the genre, and I would be hard pressed to say I have played a better strategy game, because I haven’t.

lets make this a sticky to help reduce the number of what's the difference questions..

just a quick question. does massive assault have a bigger single player missions then man?

Yes. And it has a lot more of them. MAN has 3 scenarios when MA has couple dozen (plus campaigns).