Catalan is one of several official languages of Spain along with Spanish. It is another Latin based language, so concepts like tenses and conjugation structure is pretty similar. Many people will tell you it is a mix of Spanish and French, and I feel that this is a good description in a nutshell. I don’t know much about french, but I know Catalan shares some things like the letter “ç,” and the phonetics for each letter aren’t always as straight forward as in Spanish (for example, “e” in Catalan utilizes the sounds that that in Spanish are “e” AND “a”). This is a good question and a good thing to learn, that in Spain there isn’t just Spanish. I’m only starting to learn Catalan myself, but very quickly you find a lot of cognates and patterns to spanish, to generally answer your question about how similar it is.