Like English, in general, Spanish has the same foundation and grammar principles regardless of where the person is from. What will change from region to region is the accepted colloquialism. If people choose to ignore grammar rules and basic principles, that is a different story. But, that is more of a choice than what is actually taught in those countries.

Yes, the pronunciation can be very different. The biggest difference, surely, is that for spanish people, the Spanish fro american countries sound like a joke, not serious. Also, some words have totally different meanings. For that, before to start learn Spanish, you should choose what Spanish you want to learn. Spanish from Spain, from Cuba, from Perú, from Chile...

Think of it like US and UK its very similar yet they have clear differences. The main differences you will find with spanish is mostly with vocabulary. Most of the grammar rules are the same thanks the the real academia de la lengua española.

It is in every Spanish speaking country the same language but there are differences in some words and also on the pronunciation. Similar as English in USA and UK.