What are mods in games?

Answer 1

Mods are edits in the games original coding to enhance gameplay or for a player to gain an advantage over other players making the experience unfair. Mods are illegal in some games but in others such as Call of Duty: Modern Warfare for the PC mods are encouraged and a dedicated server is running for these modded maps/weapons etc. to be played by other players.

Okay, let's make this clear. Mods aren't illegal in games and most mods add additional functionality as opposed to simply changing the existing code. Mods are almost never done at a code level anyway, but at a scripting level instead. In multiplayer games, mods would generally get you banned from servers they're not allowed in, but most games that support mods are clever enough to load mods and unload them as appropriate. Let's also be clear about Modern Warfare. Whilst it certainly supports mods, it never truly encouraged them, as it took many updates to even allow options to show modded servers in the serverlists.