The Museo del Prado - Prado Museum - is considered to house one of the finest art collections in the world. It displays thousands of European paintings, sculptures, and other works of art throughout its halls - and this is only a fraction of their collection!
What doesn't fit in the display space is stored or sent on loan to other fine galleries throughout the world. The Prado specializes in European art from the 12th-19th century (Madrid's Reina Sofia Museum is home to the post-19th century art), that was built from the Spanish Royal Collection.
The most famous piece in the collection that is on display at the Prado is Las Meninas by Diego Velazquez. He dedicated many of his own pieces to the museum and had a hand in obtaining several works from great Italian painters as well. In tribute to Velazquez, his statue is one of the few marking the entrances to the museum.