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.
Southern Europe
The most widely spoken family of languages in Southern Europe are the Romance languages, the heirs of Latin, which have spread from the Italian peninsula, and are emblematic of Southwestern Europe. "(See the Latin Arch.)"
By far the most common Romance languages in Southern Europe are Italian (spoken by over 50 million people in Italy, Malta, San Marino, and the Vatican) and Spanish, which is spoken by over 40 million people in Spain, Andorra and Gibraltar. Other common Romance languages include Portuguese (spoken in Portugal and Andorra), Catalan (spoken in eastern Spain, Andorra and Alghero in Italy), Galician (spoken in northwestern Spain) and Occitan, which is spoken in the Val d'Aran in Catalonia, in the Occitan Valleys in Italy and in southern France.