Within Minecraft, there are so many ways to be creative and build the worlds of your dreams. But when you want to extend Minecraft even further and introduce new mobs, items and other artifacts into your world, you’ll want to go to the next level of creation by building new Add-On packs that can transform Minecraft.
Add-ons are the first step toward bringing customization to all editions of Minecraft and are officially supported by Mojang/Minecraft. They allow the addition of elements to the game through a combination of resource packs and behavior packs. They are accomplished by behavior packs.Mojang provides official Creator Documentation in order to assist the community.
Behavior Packs can modify the content of the game with the ability to change entity behaviors, loot drops, spawn rules, items, recipes, and trade tables. Behavior Packs can be used to alter existing content or even create new entities, blocks and items. Behavior packs are often used in conjunction with Resource Packs to create unique experiences that wouldn't be possible in the base game. 
Add-ons are also often used in maps for the Minecraft Marketplace. All add-ons needed for the map are automatically downloaded and integrated into the world after purchase when downloading. The add-ons can be used in other maps, but it always depends on whether or not the developer allows this.
It is also to be noted that the Marketplace is the only official add-on market, but there are add-on websites that have community-made add-ons by non-Minecraft partner developers, and that those add-ons are not just piracy versions of existing Marketplace packs, but original work. Third-party add-on are nevertheless not available on PlayStation 4 and Nintendo Switch.