Gemini

Gemini (♊) is the third astrological sign in the Zodiac, originating from the constellation of Gemini. In western astrology, this sign is no longer aligned with the constellation as a result of the precession of the equinoxes. In astrology, Gemini is considered a "masculine", positive (extrovert) sign. It is also considered an air sign, and is one of four mutable signs. Gemini has been closely associated with the planet Mercury and is considered to be ruled by it. Being the third sign of the zodiac, Gemini has been associated with the astrological third house.

Individuals born when the Sun was in this sign are considered Gemini individuals. Under the tropical (western) zodiac, the Sun is in the sign of Gemini roughly from May 21 to June 22, ending at the moment of summer solstice by definition. Under the sidereal zodiac, it is currently there roughly from June 15 to July 15. The Sanskrit name of Gemini in Hindu astrology is Mithuna.

Compatibilities
Generally, Gemini is considered compatible with Libra, Aquarius and Gemini itself. These signs share an "airy" demeanor and are believed to relate on that merit.
There are many variables in the astrology chart that determine compatibility of individuals. The position of the Sun, the Moon, the planets and the aspects they form with each other are assessed by astrologers before judgment on compatibility is made. The signs listed as compatible with Gemini do not reflect an individual profile or individual reading as interpreted within astrology, but rather reflect a general guideline and reference to compatibility as dictated by variables such as Qualities and Elements within the Zodiac. The branch of astrology dealing with interpersonal compatibilities is called Synastry.
Source : http://en.wikipedia.org/wiki/Gemini