Bussard collectors are a type of technology used by Starfleet to use magnetic fields to collect interstellar gas molecules for use as fuel. Since the most common free gas in the universe is hydrogen, this is the usual substance collected. Hydrogen is then taken from the Bussard collectors and enriched into deuterium or tritium for fuel use and then Starfleet tankers are used to transport it. (DS9 reference: Star Trek: Deep Space Nine Technical Manual)
A Bussard collector which is combined wih an ignition mechanism to immediately burn fuel as it is collected for engine use is known as a Bussard ramjet.