Castilian? Isn’t that Spanish too?
Yes. They are synonyms however in some places one is used more than the other. There is a map from Wikipedia about this topic. Here there is a map of the name that is mentioned in every constitution of Spanish-speaking countries. Notice that many countries use Castillian in their constitutions, but in everyday life conversation, Spanish might be heard more often.
Where is Spanish spoken?
Spanish or Castilian is spoken in Europe, in Spain, and in parts of America, called Hispanoamerica. Spanish-speaking countries of North, Central, and South America are colored red on the map. Unknown to many people is the fact that is also present in Africa, in Equatorial Guinea, and of course also in the territories that belong to Spain: Canary Islands, Ceuta, and Melilla. In Asia and Oceania is still possible to hear Spanish in the former colony, the Philippines, and on Easter Island (which belongs to Chile).
The Green countries on the map for the Lusofonia, Portuguese-speaking countries. This language is very close linguistically to Spanish. However, they are also very close geographically: in South America and in the Iberian peninsula. Because of this common origin, the name Iberoamerica was created to includePortuguese and Spanish-speaking countries.
Finally, we can find other official languages in America: English, Dutch, and French. The blue countries on the map are the francophonie. French is a Romance language, like Spanish and Portuguese (among many others) that evolved from Vulgar Latin, and for this reason, Latin America is the group formed by the countries where we can hear any of these three languages. Some interesting fact: The only country, Equatorial Guinea, where the three languages are official is in Africa, so it is not Latin America!
How many Spanish speakers are there?
It is difficult to calculate the exact number but it is usually mentioned that it is used by over 543 million people. Below there is a map with the number of Spanish speakers by country. Source.
The Spanish-speakers often live in societies where other languages exist, are spoken and even used in their daily lives. Why does this matter? Yes. Spanish is not an isolated language. It is not a pure and unchangeable collection of words. It is not either just a dialect of the old Latin languages. The native inhabitants of the Iberian peninsula may have left some words. There have been influences from other languages first, when Castilians shared their cities with Germanic peoples, and later, with their neighbors who used Arabic language and Hebrew as their liturgical texts. When it spread all over the Americas, languages such as Taino, Guaraní, and Nahuatl had a huge impact on the vocabulary, not only for naming new ideas.