Energy riders are a species of being that exists in a state of pure energy, rather than in physical form. Unlike other beings of pure energy, such as the Organians, energy riders do not exist on a planet and -- in fact -- may never have had physical bodies. Instead, energy riders live in electromagnetic clusters in open space.
Energy riders feed off of the life force of living beings with physical bodies, who energy riders possess for brief periods. The first rule of such possessions is "do not hurt the body". The second rule is to only remain within the host for long enough to have a "taste" or "sip" of the body's energy. If an energy rider resists either of these rules, they are considered diseased and another energy rider is assigned to kill them.
Prolonged possession by an energy rider can have bizarre consequences. For example, after Chiana was possessed by a diseased energy rider for a prolonged period, she began to see glimpses of the future. This later developed into an ability to see current occurences around her in slow motion, allowing her to see even greater detail -- though at the expense of her natural eyesight.
Energy riders appear in Farscape.