Budapest is the capital of Hungary. As the largest city, it is the country's principal political, cultural, commercial, industrial, and transportation centre. In 2011, Budapest had 1,733,685 inhabitants, down from its 1989 peak of 2,113,645 due to suburbanization. The Budapest Commuter Area is home to 3,284,110 people. The city covers an area of 525 square kilometres (202.7 sq mi) within the city limits. Budapest became a single city occupying both banks of the river Danube with the unification of west-bank Buda and Óbuda with east-bank Pest in 1873.
Cited as one of the most beautiful cities in Europe, its extensive World Heritage Site includes the banks of the Danube, the Buda Castle Quarter, Andrássy Avenue, Heroes' Square and the Millennium Underground Railway, the second oldest in the world. Other highlights include a total of 80 geothermal springs, the world's largest thermal water cave system, second largest synagogue, and third largest Parliament building. The city attracts about 4.3 million tourists a year.
You may find more detailed information on the Budapest Portal, the official website of Budapest, Budapest Info, a website devoted to touristic information concerning the city or on the website of Visit Budapest.
(Contains excerpts from the Wikipedia page on Budapest).