Aisha bint Abu Bakr was the second wife of the prophet Muhammad. She was the daughter of Abu Bakr, the closest companion of the prophet and the first convert to Islam outside Muhammad's family. Muhammad spent 25 years in a happy marriage with Khadijah, a wealthy older woman. After Khadijah's death, Muhammad was in his fifties. Aisha was quite a bit younger. Many sources say she was as young as nine years old.
Needless to say, this is a controversial part of Muhammad's biography. If you do a Google image search for Aisha, as I did and I recommend you do not, you will see all kinds of vile anti-Muslim propaganda highlighting this and calling the prophet a pedophile. (So at right is a picture of actress Aisha Taylor instead.) Islamic lore even has a story of Muhammad and Aisha playing together with her toys, which perhaps was considered charming but to modern ears sounds as creepy as a child predator driving around in an ice cream van.
The important thing to know about Aisha's age is that it may not be literal or accurate, as it played a role in the power struggle following the death of Muhammad. In broad, overly simplistic terms, following this Islam split into Sunni and Shia camps. The former traces the succession of Muhammad through Abu Bakr, the latter through Ali, Muhammad's cousin and husband of his daughter Fatimah. To stress Abu Bakr's case, advocates played up the virtuousness of his daughter Aisha. Many of Muhammad's wives were war widows, but Aisha was the only wife who was a virgin when she married him. Emphasizing (and possibly lowering) her age was to emphasize her virginity and thus virtuousness. Scholars note that child brides were common at that time and place (and unfortunately linger in some primitive pockets), but others think that Aisha's real age was anywhere from 15 to 24.
Aisha survived for some fifty years after the death of Muhammad. She became a respected figure and authority on Islam. She led forces against Ali at the Battle of the Camel, but Ali triumphed and her influence gone. As a result, she is revered by Sunni Muslims, not so much by the Shia.
Aisha died on July 13, 678. She was buried at the Al-Baqi' Cemetery in Medina, the final resting place of many of Muhammad's wives and companions. The mausoleums of Al-Baqi' and other holy sites were destroyed in the early 20th century by King Ibn Saud, under the influence of Wahhabi fundamentalist doctrine, which decries the practice of "grave worship".