Mods are ways for you to tweak your UI or change looks of in-game models like Mounts, Costumes and others. Mods will only change how you see a model, and not for other people.
Unfortunately mods are only supported on the PC version.
Mod Packs are lists of mods grouped together, anyone can create one if they are logged in and can be used to help group mods you like, or follow a similar theme.
List of Mod Packs
Listing all Mod Packs.