Spanish can refer to the people who live in Spain. There are different cultures in different regions of Spain. Some, notably the Basques  and Catalans  even want to separate from Spain. The Roman Catholic Church was the official Spanish religion till 1978 but has lost influence. There are about 1 Million Spanish Muslims, mostly North African immigrants.
Spanish can also refer to the language spoken in parts of Spain and in many Latin American countries which is called Spanish. Large parts of the Spanish language are derived from Latin and some parts are from Arabic. Actually the standard Spanish language is Castilian (called Spanish) but other languages are also spoken in Spain. Basque which has no relation to any other language is spoken by the French border. Catalan (similar to Provençal) and Galician (similar to Portuguese) are still spoken and written in districts where those cultures are strong. 
Spanish can also refer to things that are Spanish like Spanish colonies.