Names given to the Spanish language
Formally speaking, the national language of Spain, the official Spanish language, is the Castilian language (as opposed to regional Spanish languages like Galician, Catalan, Asturleonese and Basque). As such both names, "español" and "castellano", have distinct and independent meanings that may be required for clarity in some specific contexts.
Generally speaking, though, both terms can be used to refer to the (national) Spanish language as a whole, with a preference for one over the other that depends on the context or the speaker's origin. "Castellano" (as well as "Castilian" in English) has another, more restricted, meaning, relating either to the old Romance language spoken in the Kingdom of Castile in the Middle Ages, predecessor of the modern Spanish language, or to the variety of Spanish nowadays spoken in the historical region of Castile, in central Spain.
Spanish language
According to 2006 census data, 44.3 million people of the U.S. population were Hispanic or Hispanic American by origin; 38.3 million people, 13 percent of the population over five years old speak Spanish at home. The Spanish language has a long history of presence in the United States due to early Spanish and, later, Mexican administration over territories now forming the southwestern states, also Louisiana ruled by Spain from 1762 to 1802, as well as Florida, which was Spanish territory until 1821.