Noob questions :D

Ok, so i've had my hero for a few months now, but i discovered this forum and now i am curious about all the things you ghuys are doing to your phones.

Firstly, what is MoDaCo Custom ROM, what changes will it make to my hero and is it worth doing?

Also what is/are 'splash screen/s'? I have seen numerous threads about creating custom ones and so forth and don't really understand what they are...

Any help/advice would be appreciated :D