Mods are third-party addons to Minecraft. These are usually installed via something like ModLoader, Forge, LiteLoader, or the Minecraft modding API (oh wait, we still don't have one).
Mods can be simple additions (such as Television), new concepts (like Craftable Spawn Eggs Mod), or extremely all-encompassing (like B N W Mod), or anywhere in between. This category is the hub for fanmade mods on the Minecraft Fanon wiki. Add your own!