Catalan was for much of its history the official speech of Aragon (which was incorporated into the larger Catalan region but retained the name Aragon because Aragon had kings and Catalonia only counts - actually the same individual). It is spoken primarily in northeastern Spain and surrounding areas (e.g. into the eastern French Pyrenees; the primary city of Catalan Spain is Barcelona). Catalan speakers have been oppressed at various times in Spanish history (as recently as under Franco), which has resulted both in the destruction of texts and in a strong tendency to conform to Spanish. Still, there are literary remains going back to about the twelfth century, and chronicles starting not much after -- and the fact that Aragon and the County of Barcelona came to be dominated by Castile, and that Catalan texts and speakers have been abused, means that there is much need for textual reconstructive work.
Catalan is another Romance language, considered by some philologists as a dialect of Occitan or more neutrally they regard Occitan and Catalan as variations of a single Catalano-occitan language. Others regard it as a separate Hispanic language.
Catalan is now once again widely spoken in Aragon - encouraged by the regional government in Barcelona, which deprecates the use of Castillian (the language that most people are taught to think of as "Spanish"). It is also still widely spoken in the Roussillon (the part of Catalonia now on the French side of the national border). It is the official language of Andorra. It is also spoken in Sardinia and in Alghero, a town in Italy, founded by the Doria family of Genoa in the 12th century, which became a Catalan colony under Peter IV, King of Aragon, in 1354. This town was the residence of Emperor Charles V from 1541. As well as being the only Catalan speaking town in Italy, it is also notable for its Aragonese architecture.