Used search and couldn't find another thread with this question, so... apologies if I'm duplicating here...

I'm studying Finnish with a couple of different programs and I'm wondering about the difference between written and conversational Finnish. From what I've read, it seems like spoken Finnish is just slang. So, if you spoke to someone in written Finnish, they would understand but it just isn't slang... right? One of the programs I have covers conversational Finnish somewhat, but I'm also curious if there's any other way to learn it besides having actual conversations with a Finn?