It's Kinda Of entertainment for us with this you don't need to buy another 2d game ever M.U.G.E.N has a lot of characters you can download by other people or yourself if you know how to make them my point is like you can get almost every single char from a 2D game into here so you don't waste money buying games when you can just download them if you can find them 1 you haven't try hard enough 2 look in more mugen websites if you need help on mugen visit the M.U.G.E.N Help section. anyways in other case:
Mugen is a free 2D fighting game engine. It allows you to create your own fighting game (with your own characters, backgrounds...). Mugen is very configurable and powerfull, you can get a gameplay as good as commercial fighting games such as Street Fighter Alpha or King of Fighters series. The only weak point of Mugen is the difficulty to create characters. But the more difficult is to begin, after some work it becomes easier, and you'll love it :). But if you don't have time or patience to make your own characters, you needn't worry: you can download characters already made by other people.
oh dammit i forgot about the stages you can also create stages or download other peoples i think i explain that??