The Jokhang is a famous Buddhist temple located on Barkhor Square in Lhasa. For most Tibetans it is the most sacred and important temple in Tibet and probably the most popular tourist attraction in Lhasa along with Potala Palace. The Jokhang temple was constructed by King Songtsän Gampo probably in 642. This temple has remained a key center of Buddhist pilgrimage for centuries. It was sacked several times by the Mongols, but the building survived. In the past several centuries the temple complex was expanded and now covers an area of about 25,000 sq. meters.

Sources, related links:
Wikipedia - Jokhang
Wikipedia - Lhasa