-GTA 4 mods, like the name suggests, modify your GTA IV game by adding new vehicles, weapons, or maps. They can be acquired from a variety of 3rd party websites as most of the mods out there are not hosted on Modmyifone.
-There is no difference between downloading a mod and buying a DLC off the PS Store. Mods do pretty much what DLCs do; add new stuff to your game.
-GTA 4 mods will only work on PC and Play Station 2 & 3 versions of GTA 4. It will not work on the Xbox 360 version, which means you need to own a PC or PS3 to be able to enjoy the game with mods.