Each Green Lantern possesses a power ring that gives the user great control over the physical world as long as the wielder has sufficient willpower and strength to wield it. The ring is one of the most powerful weapons in the universe, and can be very dangerous. While the ring of the Golden Age Green Lantern (Alan Scott) was magically powered, the rings worn by all subsequent Lanterns were technological creations of the Guardians of the Universe, who granted such rings to worthy candidates. These individuals made up the intergalactic police force known as the Green Lantern Corps.

The Yellow Lantern, aka The Sinestro Corps story:

The Parallax entity is a space parasite that is the embodiment of fear that was imprisoned within the Central Power Battery on Oa. As time passed, the entity became known as the yellow impurity, the cause for the power rings' weakness to the color yellow.

Sinestro, at the time the universe's greatest Green Lantern, was sent to Earth by Supernova in a plot to erase Guy Gardner from history. Booster Gold was assigned to prevent this from happening. To do so, he convinced Sinestro to leave Earth, claiming that he was an admirer from the future, and that his yellow Legion of Super-Heroes flight ring was a tribute to Sinestro. When asked what Corps he belongs to, Booster ad-libbed "The...Sinestro Corps," leading Sinestro to twirl his mustache in thought while mumbling "Of course...Of course."

After Sinestro went rogue, he was banished by the Guardians of the Universe to Qward in the Antimatter universe. When he returned, he wielded a power ring which used yellow energy. After various encounters with Earth's Green Lantern Hal Jordan, he was imprisoned within the Central Power Battery. There he was able to use his ring, which uses fear, as opposed to willpower, as a power source, to awaken Parallax from hibernation. From here, Parallax and Sinestro were able to influence the fall of Hal Jordan and instigate the fall of the Green Lantern Corps, leaving one last Green Lantern: Kyle Rayner.

Symbolizes: Fear
The Yellow Fire of Fear

The Yellow Lantern Corps Oath:
"In Blackest day. In Brightest night!
Beware your fears made into light.
Let those who try to stop what's right --
burn like his power -- Sinestro's might!"