Tylium is an extremely rare ore used as fuel for the Colonial Fleet once it is refined.
Tylium is a metallic ore, and is very rare throughout the known universe, but essential for fueling both human and Cylon space ships, including for the purpose of FTL jumps. Tylium is the focus of episode 10, season one, and is pronounced "till-ium", or occasionally "Tylinium".
In the re-imagined series, raw Tylium ore appears as a yellow, sand-like substance before it is refined into a golden liquid similar in appearance to motor oil; however it flows quickly like gasoline. In the episode "Dirty Hands", it is shown that the raw Tylium is stored in massive silos aboard the fleet's Refinery Ship where it is poured (by hand) onto conveyor belts and heated in ovens. It was never specified how this process actually converts the raw Tylium to a liquid state, however it is stated that "hot" Tylium is highly unstable, and if the carefully controlled process is interrupted it could "go critical" and explode.
In the episode "The Hand of God", Gaius Baltar mentions in dialog that the enthalpy of refined Tylium is "to the order of half a billion megajoules to the kilo" (the enthalpy of combustion of gasoline is for comparison 45 megajoules to the kilo, so tylium is more than 10 million times more energetic). Tylium is unstable and has a strong exothermic reaction if sufficiently heated, especially the less stable refined Tylium precursor. It is also mentioned that Tylium will become useless if subjected to strong radiation, which renders the ore inert