Catalan is a regional language spoken primarily in the Spanish Autonomous Communities of Cataluña and Valencia. Cataluña is known in English as Catalonia and in Catalan as Catalunya. It is also spoken in the Balearic Islands, in small parts of southern France, and small parts of southern Italy. It is a Romance language, just as Spanish is. Others may feel free to disagree on this point, but having studied some Catalan, I would say that it is right in the middle of Spanish, French, and Italian.
Some examples of the differences, Spanish followed by Catalan, followed by English:
Chocolate, Xocolate, Chocolate
Calle, Carrer, Street
Poco, Poc, Little
Soy, Soc, I am
As you can see, the languages are quite similar, but they are indeed distinct languages.