Wat Arun is a Buddhist wat or temple in Thailand on the Thonburi side of the Chao Phraya river directly across from Bangkok. It was made a royal wat by the king Taksin in the eighteenth century; he gave it the name Wat Jaeng, literally Temple of the Dawn. After the Emerald Buddha was captured from Laos it lived in Wat Arun for a short time, but that revered statue was moved to its present home in Wat Phra Kaew in 1785. Rama II enlarged the temple, as did Rama III, who also changed its name to Wat Arunratchatharam; Rama IV changed it to Wat Arunratchawanaram. Most Thai just call it Wat Arun and say that its name comes from Aruna, the Indian god of the dawn; in English it's sometimes called the Temple of the Dawn.
Wat Arun is featured on most tourist information about Bangkok, for it's a striking sight. The complex spreads out around a very tall Khmer-style tower or prang surrounded by four smaller prang; the whole structure symbolizes Mount Meru. Steps lead from the ground to two terraces supported by frightening demon figures; on the terraces are images of the Buddha at important stages of his life. Sources disagree about the height of the central prang: I've seen figures ranging from 75 metres to over 100. However tall it really is, the whole structure looks quite impressive towering over its neighbours, and when you get up close you can see that the buildings are inlaid with bits of broken porcelain which was apparently once used as ballast by boats coming from China.
Wat Arun is open every day from 8:30 AM to 5:30 PM, and the entrance fee for farang is 30 baht (some sources say 10 baht); Thai people get in free. The best way to get there is to take a 2 baht ferry from the Bangkok side in front of Wat Pho, or arrange a stop at the pier as part of a canal tour. The address of the temple is 34 Arun Amarin Road, Bangkokyai, and they even have a telephone and fax number (891 1149), but I can't imagine what kind of a response you'd get if you tried to phone them. Best just to show up.
For some rather nice photos of Wat Arun, see
http://bangkok.thailandtoday.com/sightseeing/08_0004_watarun.html
http://www.thaistudents.com/guidebook/watarun.html