One of the features of Xbox live is the marketplace is that you can buy stuff like themes, map packs, gamer pictures etc using microsoft (MS) points. MS points is the Xbox live currency where certain amount of real cash is turned into a certain amount of MS points. The good things bout this marketplace is that you can buy whatever that is available.

The bad thing about this marketplace is that almost everything that is available costs something. Want a Halo 3 theme? Need to pay about 150 MS points. Want the famous horse armor for Oblivion? Another 150 MS points (the best part is, the pc version is cheaper and you can always look around for that patch to install that horse armor). Need more maps to wage war on? Another $$$$ MS points. Want street fighter 2? Around US10 I think, which is quite steep considering there are FREE street fighter 2 software on the net.

There are some items that are free however, like the game demos (duh) and occasionally some good developer put in some stuff to enchance the game for free (like Sega's Chromehounds). Unfortunately there are some developers out there who like to milk their gamers... too many to mention.