Split Kings are, as the name implies, split into two separate bases, one that controls each side of the bed.
In order to accomodate this, Split King mattresses are actually made up of two separate mattresses (usually Twin XL), that are laid side by side and connected in the middle in some way.
Split King beds are perfect for couples who have different needs. If one partner likes to sleep with their head elevated, and the other doesn't, a Split King bed has your back. If one of you likes to read or work on your laptop in bed, but the other one needs to get up early, a Split King will allow you to sit up and read while your partner snoozes away.