Impa is a caretaker to Princess Zelda. The original Impa is portrayed as an old woman who calls for Link to save Zelda from Ganon and his henchmen. The Impa of Ocarina of Time is a Sheikah, and the guardian of Princess Zelda, as well as being a descendent of the tribe that founded Kakariko Village. It is later revealed that she is in fact the Sage of Shadow, one of the Seven Sages.
In Twilight Princess, a character named Impaz dwells in a village which Hylian text suggests is called "Old Kakariko Village", and claims both that she is serving the Royal Family and that she was named after the founder of the village.
In Skyward Sword, Impa is a member of the Sheikah assigned to protect Zelda during her time on the surface. She appears as both a young woman and as the elderly overseer of the Sealed Grounds.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.