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.
Salvadoran Spanish
Salvadoran Spanish is geographically defined as the form of Spanish spoken in the country of El Salvador. The Spanish dialect in El Salvador shares many similarities to that of its neighbors in the region, but it has its stark differences in pronunciation and usage. Salvadoran Spanish is the only Spanish dialect in North America to be usually called "castellano", instead of "español". El Salvador, like most of Central America, uses voseo Spanish as its written and spoken form, similar to that of Argentina. Vos is used, but many Salvadorans understand tuteo. Vos can be heard in television programs and can be seen in written form in publications. "Usted" is used as a show of respect, when someone is speaking to an elderly person.