Vespene gas

Vespene gas (or just "gas" for short) is the resource used to produce advanced units for all races (the other being minerals). It is green in color and emerges from the top of volcano-like structures called Vespene Geysers, like the one shown.

According to Doran Routhe, a famous Terran scientist, solar systems outside of Sol's could contain new resources. At least two have been discovered in the Koprulu Sector, Vespene and blue mineral crystals.

Mining / Gathering
In order to transport and store the Vespene gas, it must first be converted into a liquid form. Each race has a type of structure for performing this conversion: the Terran Refinery, the Zerg Extractor, and the Protoss Assimilator. Each geyser has a fixed amount of gas readily available (usually 5,000), and it is collected 8 units at a time and brought to the base by a worker (Terran SCV, Zerg Drone, or Protoss Probe); only one collector unit can be inside the building collecting at any one time, but multiple workers can be queued up to mine from it (usually no more than 4). Once the fixed amount of gas is expended, the source is said to be depleted after which only 2 units of gas are collected at a time.

In StarCraft II, the mechanism may change to allow for "higher-yield" Vespene geysers.

Usage
Generally speaking, the more advanced a unit is and the more abilities it has, the more Vespene gas it requires to produce. Flying units and spellcasters (any unit with an energy meter) require Vespene to create. Basic structures do not require Vespene gas to construct (except the Hydralisk Den), while advanced structures usually do. Terran units also require Vespene gas to repair units that were built with Vespene.

Vespene is widely sought after for different reasons. The Terran use it as an energy source, in a manner similar to the way we use fossil fuels now. The Protoss use it as a catalyst for their psi-driven machines, and the Zerg use it for metabolic purposes and as a method of inducing evolutionary enhancements. It has been rumored that exposure to raw Vespene is responsible for the Ultralisk's improved biological carapace.

Beginning
Because a player with no Vespene gas should be able to construct a base (and players start with 50 minerals and no gas), the workers, the base structure ("town hall"), and the Vespene gas converter structure never require Vespene gas to construct. Each race also has a simple ground soldier that requires only minerals to produce, the Terran Marine, the Zerg Zergling, and the Protoss Zealot.

Storyline / Campaign
Vespene and mineral formations slowly regrow over time on planets such as Antiga Prime; this allowed the Terran to survive in the Koprulu Sector.

It is unknown if Vespene gas has any relation to Terrazine gas, a potent psionic reagent, or to chlorine gas, which occurs in small amounts in volcanic gases along with hydrogen chloride.