A warp bubble is a region of space time enclosed in a fold, or bubble, of highly curved space. By expanding the space time metric behind the bubble and contracting the metric in front, the bubble can be made to move without the use of propellant mass. The vessel can apparently be coupled to the warp bubble(s) in various ways; by containing the bubbles wholly within the ship, as in the Displacement Drive; in front of the ship, coupled by gravity and/or magnetism, as in the Halo Drive; or entirely enclosing the ship, as in the Void Drive.
Note that no warp bubble has ever been observed traveling at super-luminal speeds. It is thought that such faster-than-light travel is impossible with void bubbles, because of dynamical instability of the warp metric at speeds greater than light and a suspected high flux of Hawking radiation that would turn anything inside a faster-than-light void bubble into a plasma of fundamental particles.