With more than 500million speakers, Spanish is the fourth most commonly spoken language in the world. Also there are 40 million Spanish speakers in the US. Because Spanish is nearly phonetically perfect, you can look at almost any word and know how to pronounce it.
In Spain this language is generally called español (Spanish) when contrasting it with languages of other countries, such as French and English, but it is called castellano (Castilian, the language of the Castile region) when contrasting it with other languages spoken in Spain, such as Galician, Basque, and Catalan.
Some philologists use Castilian only when speaking of the language spoken in Castile during the Middle Ages, stating that it is preferable to use Spanish for its modern form. The subdialect of Spanish spoken in northern parts of modern day Castile is also called Castilian sometimes, and differs from those of other regions of Spain, however the Castilian dialect is conventionally considered in Spain to be the same as standard Spanish.
The name castellano is widely used for the language in Latin America. Some Spanish speakers consider castellano a generic term with no political or ideological links, much as “Spanish” in English.