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.
Catalonia
Originating in the historic territory of Catalonia, Catalan has enjoyed special status since the approval of the Statute of Autonomy of 1979 which declares it to be "Catalonia's own language", a term which signifies a language given special legal status within a Spanish territory, or which is historically spoken within a given region. The other languages with official status are Spanish, which has official status throughout Spain, and Aranese Occitan, which enjoys co-official status with Catalan and Spanish in the Val d'Aran.