Can someone explain what "Buddy Press" Themes are?

I see,, as well as buddy press themes from WPMU Dev.

1. I’m just curious what makes Buddy Press themes different, unique, special, etc.?

2. Is there a difference between Buddy Press themes offered by WPMU and ?? (Or do they all follow the same structure?)

3. I’m just confused as to what makes a theme a BuddyPress theme?

4. What type of website or client should use a “Buddy Press” theme vs any other type of WordPress theme?