The nacelle is an outboard structure on starships that houses the warp coils. The warp coils create multiple subspace displacement fields, which warp subspace around the vessel, allowing it to "ride" on a spacial distortion and travel faster than the speed of light. Modulation of these multiple fields - the interaction between a vessel's warp generator and normal subspace - affects a ship's speed and course direction.
The warp coils are fed by plasma conduits from the vessel's warp core reactor assembly. Nacelles are usually separated from the main structure of the ship because radiation generated by the nacelles, when at optimal levels, could be deleterious to the safety of ship and crew. Nacelles are separated from the ship by large pylons and usually house a Bussard ramscoop at the fore end for the purpose of collecting hydrogen fuel.
A nacelle's interior also includes a small control room, accessible in nominal conditions by a Jefferies tube that permits maintenance and monitoring of the system's operation.
Most vessels typically have two nacelles. Experience has shown that having two nacelles is the optimal configuration for vessel control and efficency but it is not unheard of for vessels to have additional nacelles or, indeed, none. One example would be Federation Prometheus class starships, which have four nacelles in standard configuration but six in total, two of which are only deployed during multi-vector assault mode.
Nacelles also have a seperation system in the event that the ship is damaged and unable to retain the nacelle safely on the pylon. On Galaxy class starships explosive charges seperate the nacelle and lift it up and away at 30m per second. If a nacelle is lost during warp flight, it can tear the ship apart as the loss of a nacelle would cause different areas of the ship to travel at different speeds. Even at sublight speeds serious impacts from weapons or other objects can spell disaster for a ship because of a feedback of energy throughout the vessel.
Some classes, such as the Defiant class and the Steamrunner class, possess nacelles that are integrated into the ship's main structure without pylons. Other races employ vessels that have no discernible nacelles as currently found on Starfleet designs, such as ships of the Klingon Bird of Prey line and the Cardassian Groumal class freighter.