From Galaxy Attack: Alien Shooter Wiki

Traps are additional challenges that can occur on a wave, independent of the actual wave itself. Traps can be sent to another player in PvP by collecting a Galaxy or Predator ship in-game, or they can occur on more difficult levels, in particular within Events.

The main types of traps, that can be triggered by Galaxy Ships and also appear in Events are:

The next set of traps can be triggered by Predator Ships:

The last set of traps can be triggered by Wings: