Geass is a special ability that is usually acquired by making a special contract with someone who possesses it.
Geass is an ability that is only limited to the user's eyes. It has been noted that an eye infused with geass will turn a slight burgundy color when used.
There are many different geass abilities which differ from person to person. One of those powers being that the user can command people to do whatever the user pleases. The drawbacks to this power however are that the user must make direct eye contact with someone and that once used cannot be used on the same person again.
Known Geass Users
Lelouch vi Britannia
Lelouch's Geass allows him to give any command to anyone and they'll obey it with direct eye contact. One downside is that it can only be used once on each person unless the persons command is removed with a Geass Canceller.
Mao's Geass allows him to read minds within a 500 metre radius. Although Mao cannot turn off his Geass which is a downside in crowds as each person's thought will be picked up in a crowd. Mao can also concentrate on a specific target to probe deeper into their thoughts and memories, which decreases his effective range.
Charles zi Britannia
Emperor Charles's Geass allows him to freely alter an individual's memories at will; he may also seal the ability of a Geass user as well as physical abilities such as eyesight. Charles soon gave up his power of Geass with V.V. for immortality.
Rolo's Geass allows him to temporarily suspend the subjective experience of time for all individuals within a given sphere of influence, effectively paralyzing anybody within its area of effect. This power doesn't need eye contact, and can be used on the same person more than once. The weakness of Rolo's Geass is that everytime he uses it, his heart stops, limiting its activation to short bursts.