Castilian is from Spain. The Spaniards went into the Americas back in the day and now we have Latin Spanish of different varieties; Argentinian, Chilean, Mexican, Colombian, etc. They're mostly the same, but with some differences. Fun-fact: In Spain they usually call Spanish as _Castellano_, but outside it's normally Spanish as it's better known of a word than Castillan/Castilian.

@impakt that is mostly correct. to be more specific, the difference between Castilian and Spanish is political correctness or rather imo the political agenda. what people worldwide refer to as Spanish is actually Castilian which is the common language of Spain, this is the language that former colonies normally call Spanish. however inside Spain, several autonomic communities (equivalent to provinces in Canada) have secondary official native languages which they promote to different degrees proportionately to the strength of pro-independence movements. for instance, if you go to the official site of the Barcelona city government you will be given an option to choose between Catalan, CASTILIAN and English for the display language (in that order), but if you go to a website from the central government you will be given an option to choose between SPANISH, Catalan, etc. fun fact, In Venezuela where I grew up the native language class in school is called Castilian as it is the academically correct thing, even though people would virtually always refer to their language as Spanish in regular conversation and documents Another fun fact, ironically for some reason over the years it became common to call the Spanish dubs made in Latin american as Castilian in order to differentiate them from those with Spanish accent even though for the general public it would make more sense to refer to them the opposite way given how the Spanish folks are more likely to call their language Castilian than the Latin american

Came out of sheer curiosity, stayed for the lesson. Cheers.

Source please...