(nooby answer) on a flash you can run 'copied' xbox 360 games, so you can just burn your own game from your computer and play it or buy copied games, you can also mod some games with an iso mod (cod5 etc.) most xbox drives are not too hard to flash. a jtag is a lot more complicated and allows you to use an .xex launcher to load certain parts of games (please somebody correct me if i'm wrong there). With this you can mod games with decrypted files and stuff that you wouldn't be able to mod with an iso mod (MW2, halo 3, most new games etc) but once again this is A LOT more complicated and you should look for a detailed tutorial or at least read up on jtagging before you go and buy one.