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.
Political divisions of Spain
The official names of the autonomous communities can be in Spanish only (which applies to the majority of them), in the co-official language in the community only (as in the Valencian Community and the Balearic Islands), or in both Spanish and the co-official language (as in the Basque Country, Navarre and Galicia). Since 2006, Occitan—in its Aranese dialect—is also a co-official language in Catalonia, making it the only autonomous community whose name has three official variants (Spanish: "Cataluña", Catalan: "Catalunya", Occitan: "Catalonha").