Addons are mods that use the physics of Valkyrien Skies and add their own blocks, items, and controls. They can vary greatly in gameplay, style, and complexity. Many are developed by third-parties. Eureka and Interactive are the only two addons that are maintained by the Valkyrien Skies team.
Addons use the API to leverage the functionality provided by Valkyrien Skies. Some mods use the API to add additional compatibility or integration with Valkyrien Skies, but don't require it to run. Addons, on the other hand, rely on Valkyrien Skies to provide their core gameplay and don't function without it installed.
Some addons are designed specifically to integrate Valkyrien Skies with another mod. Interactive and Clockwork, for example, both integrate Valkyrien Skies with Create. Integration addons add new features and gameplay elements, enhancing your experience. They are not, however, necessary to use. Valkyrien Skies and Create work together just fine without Interactive or Clockwork installed.