it may seem harsh but the bottom line is that you do need to read the documentation - it is VERY comprehensive and will answer any queries you have. Your question was too open-ended to be answered in a post and the very best answer that anyone could give was to read the docs.
A short answer is that Dreamcast is one of the many systems that works with Pi, but you do need to understand what a BIOS is and why it is needed. When you understand this then you will be able to make loads more emulators work properly by yourself, and every little bit of knowledge will make using retropie easier.
Using a Pi isn't like switching a phone on and everything works great with little interaction, one of the best things about it is that it requires effort to learn, you will get masses of help here but firstly need to try and help yourself.
As for a list of supported emulators, there is one here...