Perhaps this is nothing new to you: Catalan is its own language and not a dialect of Spanish, even if this was assumed for a long time by linguists. This fact must be remembered before beginning the study of Catalan. Even if you can already speak Spanish, learning Catalan means learning a whole new language.
Catalan is along with Spanish classified as a Romance language. Both languages also belong to the subgroup Ibero-Romance languages along with Portuguese and Galician. Inside this group Catalan is often viewed as a special case as a bridge between the Ibero-Romance and the Gallo-Romance groups of languages. The Gallo-Romance languages include French, Provencal and Occitan.
Why are we discussing the finer points of the sub classifications of the Romance languages at such length? Because this knowledge could be useful in learning and understanding Catalan. Why? The answer to this question is: The languages of the Ibero-Romance Group are very different in terms of pronunciation and vocabulary. The grammars of these languages are in many instances the same or at least very similar. That means that if you already speak Spanish, Galician or Portuguese you will often encounter familiar grammar when learning Catalan. If however if you learnt French in school you will quickly notice connections between Catalan and French. As mentioned Catalan is a bridge between the two sub groups of languages and it has a lot of vocabulary in common with Occitan and therefore with French.
When learning always use the opportunity to find connections between languages to help you remember new words. You will learn faster and will have the advantage of being able to learn other languages more easily.
The distribution of Catalan: The Catalan language is spoken on the western edge of the Mediterranean. About 10 million people will able to understand you if you work through the Catalan course.
The distribution of Catalan extends beyond Spain: Catalan is spoken in Italy (in the city of Alghero on the coast of Sardinia), in France (in the Pyrenees) and in Andorra (where it is an official language). In Spain Catalan is an official language in the two autonomous regions of Catalan and Valencia. It is also spoken on the Balearic Islands (Mallorca, Menorca, Ibiza and Formentera).
As already written Spanish and Catalan are two distinct Ibero-Romance languages.
What differences and what similarities are there? The two languages are quite different in their pronunciation and vocabulary. If you already speak a bit of Spanish or Portuguese we would advise you to pay particular attention to the pronunciation of Catalan.
The grammar of the two languages are often the same or at least very similar. This means if you already know some Spanish, Galician or Portuguese, then you will encounter during your study of Catalan much familiar grammar.
If Catalan is the first Romance language that you learn, it will help you to learn other Romance language more quickly.