¿Español or Castellano?
Castilla was one of the kingdoms that eventually became what we now know as Spain.
What is known as Spanish is actually the language that was spoken in that particular kingdom (derived from popular latin, is a romance language).
In Spain and several of the former american spanish colonies, we refer to our language as "castellano" which is a more specific name than the general "spanish". From what I have read, the latter was adopted in america to identify the language spoken by those conquistadores who "came from Spain".
Keep in mind that you can hear several other languages spoken in Spain, such as vasco, catalan, gallego and others. But only Castellano is known as "Spanish".
So, colloquially, you might hear "castellano" very often (even though the term "español" is more widely spread, specially among non native speakers), when visiting the aforementioned countries!