Tali'Zorah nar Rayya is a quarian and a member of Commander Shepard's squad. She is the daughter of Rael'Zorah, a member of the Admiralty Board. Though young, Tali is a mechanical genius.
During the events of the first game, she is on her Pilgrimage, a rite of passage to prove her worth and bring something of value back to her people aboard the Migrant Fleet. Early in the second game, having completed her Pilgrimage, Tali has been entrusted with leading an important research mission for the Admiralty Board.
Tali was born in 2161 aboard the Rayya. Having reached maturity, she departed the Migrant Fleet on her Pilgrimage. On her travels she began hearing reports of geth, who had never ventured beyond the Perseus Veil since driving her people into exile, and became curious. She tracked a patrol of geth to an uncharted planet, waited until one was separated from its unit, then disabled it and removed its memory core. She recovered a file from its audio banks that revealed Saren Arterius was behind the attack on Eden Prime, but possessing the file put her in grave danger. She was pursued by Saren's forces, and escaped to the Citadel.
Source: Mass Effect Wikia, "Tali'Zorah nar Rayya," available under the CC-BY-SA License.