Each gem is smooth and oblong, and about the length of a man's hand. They can shrink to fit into the groove on the Infinity Gauntlet, where they could formerly act in concert to grant the wearer absolute power over all of reality.
The six gems are:
Time Gem (red) - Control over the flow of time.
Space Gem (purple) - Control over space, allowing instantaneous travel across vast distances.
Power Gem (pink) - Increases physical strength and endurance.
Soul Gem (green) - Control over the soul.
Mind Gem (blue) - Allows mind control and vastly enhances the holder's intellect.
Reality Gem (orange) - Control over reality itself, creating and uncreating matter and energy.
The Infinity Gems figured prominently in three major crossovers. In chronological order, they were:
Following the Infinity Gauntlet event, the Living Tribunal declared that the Infinity Gems would no longer function as a whole, although Eternity (the incarnation of that cosmic concept) may be able to reverse the decision should he desire to. When the gems were separated, they were distributed to individual members of the Infinity Watch.
In the recent JLA/Avengers crossover, the DC Comics villain Darkseid obtained the full Gauntlet, but casually discarded it when he discovered it wouldn't work.
In it, Thanos steals the gems from the Elders of the Universe in possession of the gems.
Following the Infinity Gauntlet event, the Living Tribunal declared that the Infinity Gems would no longer function as a whole, although Eternity (the incarnation of that cosmic concept) may be able to reverse the decision should he desire to.
A cross-over story arc Ultraforce revealed that the seven gems were originally extracted from the consciousness of a cosmic being known as Nemesis.