Add-Ons are features that are currently only available in Pocket Edition and Windows 10 Edition. Add-ons make changes to the appearance or behavior of various elements of gameplay in Minecraft. Items, Blocks, objects, Mobs (including their behavior), can all be modified through add-ons.
Two example add-ons have been provided by Mojanghere. These two examples include an "Alien Invasion" (which was showcased at MineCon), and a "Castle Siege" add-on.