Sometimes you’ll run into people who refer to what we call Spanish as castellano (Castilian) instead of español (Spanish).
To understand the term Castilian (castellano) we need to understand that Spanish is primarily derived from Latin, which arrived on the Iberian Peninsula (the peninsula that includes Spain and Portugal) around 2,000 years ago. On the peninsula, Latin incorporated some of the vocabulary from other regional languages and then became known as Vulgar Latin.
For reasons more political than linguistic, this dialect (Vulgar Latin) was used commonly in what is now the north-central portion of Spain, which includes Castile. Later it spread throughout the region.
In the 13th century, King Alfonso pushed for historic documents to be translated into the regional dialect, known as Castilian (castellano), which helped Castilian evolve to become the standard language. He later made this dialect the official language for government administration.
Another factor that helped spread Castilian (castellano) as an official language was the expulsion of the Moors. Castilian became the official tongue of Spain. Later efforts to educate people on the use of Castilian by Arte de la lengua castellana by Antonio de Nebrija, generated the first Spanish-language textbook and one of the first books to systematically define the grammar of this European language.
Even with the growing popularity of this language, this didn’t eliminate other Peninsular languages such as Galician, Euskara (or Basque) and Catalan. Even today all of these languages coexist in Spain.
The term Spanish (español) encompasses the entirety of the Spanish language including not only Spain but Latin America, The Caribbean and Equatorial Guinea in Africa (where believe it or not Spanish is the official language!)
So basically, the main difference between the two terms is that Castilian (castellano) is the original dialect that generated what would later become, and what we know today as, Spanish (español).
Nowadays, the term Castilian is used in other ways too. Sometimes it is used to describe the language that is spoken in the North-Central region of Spain. In the Northwest region, for example, Galician is the language. In the Northeast region, Catalan is the language. Basically, the term Castilian is now used to specify the language spoken in a specific region.
However, the absolute authority for Spanish – in terms of language – is The Royal Spanish Academy. Nowadays, the Real Academia considers Spanish as a universal language. Thanks to Wikipedia we can see the top 3 most spoken languages in the world below:
|Language||Native name||Native speakers (millions)||% of world population||Mainly spoken in||Notes|
|Mandarin||官話/官话||955*||14.4%||China, Taiwan, Singapore, and Malaysia||Part of Chinese language family|
|Spanish||Español||405*||6.15%||Argentina, Bolivia, Chile, Colombia, Costa Rica, Cuba, Dominican Republic, Ecuador, El Salvador, Equatorial Guinea, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Puerto Rico, Spain, United States, Uruguay, Venezuela, and Western Sahara. See List of countries where Spanish is an official language||Partially mutually intelligible with Portuguese and Italian|
|English||English||360*||5.43%||United Kingdom, United States, Canada, Australia, Ireland and New Zealand. See List of countries where English is an official language|
As you see, Spanish is the 2nd most spoken language in the world. In general, the terms castellano or español can sometimes have political implications. In fact, in some parts of Latin America, the Spanish language is known routinely as castellano rather than español.