Cloud backup

This is a very stupid question I'm sure, but I can't find a proper explanation online and I need to be sure. I recently almost lost all of my files, so now I'm backing up in more than just the one way I used to (via external hd). I never used the cloud but now I'm wondering if, whether I sync all my documents to the cloud, does this function as a type of extra backup for my documents? So if say my hard drive crashes, I will still be able to access my documents from elsewhere? Also, does this sync process of new documents happen automatically?

Thank you!