Barcelona is the capital of Catalonia and the second largest city in Spain. It has a rich history dating back to the 3rd century BC and contains architectural treasures from various eras. The document outlines Barcelona's main attractions such as La Sagrada Familia church, Park Güell, Las Ramblas boulevard, and several buildings and works by architect Antoni Gaudi. It also mentions the beaches, Olympic venues, and museums as top things to see and do in Barcelona.