Initially presented as enemy shock troops in the original Half-Life, Gordon Freeman's actions in that game freed the Vortigaunts from their enslavement at the hands of the Nihilanth, and they quickly became allies of humanity, joining their resistance movement against the Combine at the behest of Dr. Eli Vance, leader of the resistance. In subsequent games they can be seen aiding the resistance forces and the player in various ways.
The Vortigaunts have a long, involved history with the antlion species, and antlion husbandry was once an ancestral practice of many Vortigaunts. Many Half-Life fans infer from this that the two are likely from the same home world. Vortigaunts use antlion larval extract in various cultural rituals, as it is apparently a powerful medium for the manipulation of the "Vortescence", a term they use to describe an energy field that apparently spans universes, and is a possible source for the Vortigaunt's energetic and healing abilities.
Vortigaunts have the ability to generate a potent form of energy, which can be released in several different ways, allowing them to project powerful bolts, create localized shockwaves, power broken generators and heal serious injuries. Further, in low doses it seems to be able to energize Gordon Freeman’s HEV Suit to some extent.
The Vortigaunts appear to have a complicated culture centered around the Vortescence. While never explained directly, the Vortescence could be considered an analogue to The Force from the Star Wars franchise, a permeating energy field that affects the entire universe, where some (like the Vortigaunts) are attuned to it, but others (Humans) are not. Additionally, Vortigaunts seem to be able to heal others more effectively as a group than on their own.
American actor Louis Gossett, Jr voiced the alien species in Half Life 2. Gossett was replaced for Half-Life: Episode Two by actor and producer Tony Todd.
Episode Two set to expand the Vortigaunt background with various character development aspects with protagonist Gordon Freeman. These outlines were later dropped as a decision had been made to include a simplistic story with a single supporting character.