"You require more Vespene Gas." -Judicator Aldaris
Vespene Gas is a prized natural resource found in abundance in the Koprulu Sector, often used as a fuel source and in the production of advanced military technologies.
In its natural state, Vespene takes the form of lime-green gas clouds. Vespene nodes are exploited by gas mining facilities built atop a Vespene Geyser. Unlike mineral fields, Vespene Geysers can be harvested indefinitely. However, after a certain amount of gas has been collected, the geyser will be "depleted". Trace amounts of Vespene will still be present, but harvesting a depleted geyser will yield very little gas. After being processed into a usable form, it is usually seen as an emerald-green liquid or gel.
Terrans: Fortunately for the humans, Vespene Gas can be used in combustion engines as a replacement for the fossil fuels they lacked. In addition to its role as fuel and building material, Vespene can be synthesized to create a substance that has an interesting effect on Terran Ghosts...
Terrans harvest Vespene from Refineries and store it in oil drums.
Zerg: The Zerg use Vespene as a catalyst to accelerate evolution, enabling them to enhance their warrior breeds. The Zerg tend to use more Vespene in their tech-tree than Minerals, so it is good to snatch multiple Vespene Geysers quickly.
Zerg harvest Vespene from Extractors and store them in Vespene Sacs.
Protoss: Protoss units and buildings are far more expensive than those of the Terrans or Zerg. It is a good idea to stock up on Vespene for the Advanced Tech-Tree, as they use far more Vespene than the corresponding units of the other races (for example, a Protoss Scout costs 100 Vespene to produce).
Protoss harvest Vespene from Assimilators and store them in Vespene Gel Cubes.