There are two words for 'yes' in French. The main one is 'oui', usually pronounced 'oo-ee'. Sometimes French speakers pronounce this word 'oo-ay' in a more casual fashion, in much the same we as we might pronounce 'yes' as 'yeah' . 'Si' has a different function, and is used to contradict a previous negative statement. It's the equivalent of the English contradictory 'Oh, yes, I am!' or 'Oh, yes it is!' . For example: 'T'es pas fatigué, toi?' 'Si!' You're not tired, are you?' 'I am!

And if you watch films from Québec, you will also hear "ouin" [WAIn] and "ouais" [WÈ]. It's just for fun and effects. For example, if I want to say "oh really!" I would say "ha ouin!" It's all just variation of "oui", "ouais" being he most casual. For some grown up men, "oui" [oo-ee] is too childish! We even use "yep" and "yup" as another variation, obviously borrowed from the English speaking world around us.