Drones are small sub-ships that you can obtain, which fire constantly alongside your ship to increase your firepower. You can have up to two drones at any time. If you collect another drone whilst you already have two drones in place, the drone you picked up first will be replaced by the new drone.
List of Drones
Drones also each have a number of skins. Once unlocked, you can set this skin as your default for that drone, so whenever you pick the drone up in a game, you have the skin version rather than the base drone. Skins offer additional features over the base drone. Usually they are more powerful, but often they will have other skills such as PvP attacks.
You can specify a different skin for PvP and single player modes, by clicking on the Use PvP icon next to a drone. This will then use that skin in PvP mode, and the In Use skin for single player.
Warning: Sometimes it is possible to unlock or purchase a skin in the game, before you own the base drone. In this situation, you will not be able to use the skin until you have unlocked the base drone.
S1 drones are the same as the base drones, except they are more powerful.
S2 drones are equivalent to Galaxy Ships, in that they will send attacks to opponents in PvP.
S3 drones are the same as S1 drones, but a little more powerful.
S4 drones are the same as S2 drones, but the PvP attack is more challenging.