A geisha is a woman trained as an entertainer in the Japanese tradition. Geisha are taught to sing, play instruments, dance, compose and recite poetry, pour tea and other drinks, etc. Men paid money to spend time with the geisha; generally, the transaction did not include sexual services. In former times, some geisha had official sponsors in addition to their ordinary customers; a geisha acted as a sort of concubine for her sponsor, while continuing to work at the geisha-house.
Traditionally, girls began training as geisha when they were as young as four or five years old. Today, they begin as teenagers, and become full-fledged geisha in their early twenties.
Geisha became popular in the late eighteenth century. They lost status during the early twentieth century, as the line between geisha and prostitutes blurred, but today the profession is respectable again. Geisha in Japan's cities are hired to entertain both local businessmen and tourists.
The word "geisha" comes from gei meaning "arts" and sha or "person" - so literally, a geisha is simply an artist. It also has the connotation of an "accomplished person."
See a gallery of geisha images here, both photographs and woodblock prints.