if you mean modding Xbox 360 games you need to download Hex Editor and if your a noob Modio and possibly Halo 3 Toy Box and you need to purchase a data transfer cable from Microsoft (1-800-4MY-XBOX) and order that for I think $14 then buy of Amazon.com or something a devise called XSATA.
If its a PS3 Download Hex Editor and get a USB double side cable or a USB flashdrive.
mods, mods, mods
moddb.com is good for mods. It has mount and blade mods and mods for virtually every other game.
you can get minecraft mods and textures off the minecraft mods website.
you can easily get mods from planetminecraft
The Mods was created in 1964.
look what ever game you want and add the word mods to the end on google. e.g oblivion mods, GTA mods
The easiest way to explain what the mods were is to watch the film Quadrophenia 1960s Mods & Rockers film
All mods are 'legal'.
Marvel mods
That depends on what mods you're talking about.
Get all the mods!
You can't get mods on the Xbox 360 because it can't download mods at all.