The power of Geass is a mysterious power imbued by "witches," such as C.C. and V.V. The powers vary, but usually they require eye contact with another individual to activate - but not always. When a person first gets their Geass power, it almost always manifests in the left eye. One exception, however, is Rolo, whose Geass was in his right eye - this is possibly because he received his powers from V.V., who appears to manifest the symbol in their right eye. Those who reach the peak of their power manifest the Geass symbol in both eyes. At this level, the vast majority of uses lose control.
Prolonged use of Geass typically takes a toll on the user. They will exhibit growing signs of insanity, or a general lack of a grasp on reality. Very few people were able to resist this after reaching the final stage.
When one is under the influence of Geass, their eyes are surrounded by a red outline for as long as they are under the affect of Geass. Those with powers that direction tamper with a person's mind will cause their victims to forget ever being affected as well as anywhere from seconds to minutes before / after activation. This would include Lelouch (commands), Marianne (possession), and Charles (rewriting memories).
All characters, unless otherwise specified, require eye contact in order to work.
Bismarck Waldstein - He can see the future when his Geass is active, allowing him to see where his opponent will strike. This makes him an almost unbeatable opponent in Knightmare and ground combat. He can, however, be overwhelmed. Does not require eye contact.
C.C. - Before becoming immortal and gaining the ability to grant Geass herself, she had the ability to make anybody love her unconditionally. Does not require eye contact (at least by final phase).
Charles zi Britannia - He can alter the memories of his victims. He can even repress another person's ability to use Geass. The memories, however, can be brought back by a "witch," like C.C.
Lelouch vi Britannia - Lelouch possesses the ability to compel anybody to do anything he desires. He can order them to answer questions, do tasks, commit suicide, and even cause the victim to order genocide. The victim can resist for a very short time, but they always break, despite their willpower.
Marianne vi Britannia - She can transfer his consciousness into another person. She was able to do this to escape death.
Mao - He has the ability to read minds within 500 meters. He has two weaknesses: He can be distracted, and he cannot shut off his power, plaguing him with thoughts all the time, driving him mad. Does not require eye contact.
Rolo - He has the ability to "stop time" in a localized area. However, he does not actually "stop" time. But, rather, he causes his victims to stop. He cannot stop a physical object, like a bullet, from traveling - he can only stop the shooter from firing. Every time he uses this power, his heart stops, meaning that he risks his life whenever he uses his Geass. He usually uses this power for assassinations. Does not require eye contact.
Jeremiah Gottwald - Jeremiah is not a true Geass user. Rather, he is a cyborg who was given the ability to cancel the affects of Geass. He was capable of removing Lelouch's mide wipe from one person, as well as freeing himself from Rolo's time stopping ability.
Immortals are former Geass users who possess the immortal code from the person who originally gave them their powers.
Unnamed Nun - Gave her code to C.C.
C.C. - Received her code from an unnamed Nun. C.C. intended to pass on her code to somebody else in order to allow herself to die, but she never did. Gave the power of Geass to: Lelouch vi Britannia, Mao, and Lady Marianne.
V.V. - Received his code from an unknown individual. Had his code taken by Charles zi Britannia. Gave the power of Geass to: Charles zi Britannia, Rolo, and many unnamed individuals in the Geass Order.
Charles zi Britannia - Took his code from V.V. Never passed on his code. Gave the power of Geass to: nobody.