EDIT: As suggested by L.Dutch, I turned the original post into a series of its own.
A "Hive Mind" is an aggressive type of shared consciousness (for more information on shared consciousness, check out this question). It's basically a group of individuals connected by a "psychic link". The main difference when compared to the shared consciousnesses is the existence of an hierarchy composed of a single "leader" and his multiple "drones".
Though the link connects every member together, it works in favor of the leader. He can access any information ever stored by his drones (again, thoughts, memories, emotions, senses, etc) and they can do nothing to stop him, since he's able to completely control their bodies.
Aside from telepathy, the leader also has an extraordinary brain capacity. His brain works as a multiprocessor, which has enough "threads" to control several drones at the same time, giving equal attention to them. Usually, a stronger brain means a higher drone count under the clutches of the leader.
As stated before, a hive mind can only have a single leader. You are free, however, to decide if the position of leader is subject to change.
How could this creature ever evolve and how is it able to estabilish such a powerful domain over others?
Stuff to help you (hopefully)
- First of all, try not to confuse hive mind with shared consciousness. I have created another question (link) to address that. I recommend you read it in order to know the difference;
- Though it seems that I was specifically referring to humans, feel free to propose a hive mind capable of assimilating any lifeform avaiable;
- If allowed by the leader, a drone can control another drone's body. But keep in mind that this "multithreading" should take a toll in a mind that isn't as developed as the leader's;
If other questions arise in the comments, I'll edit the post and keep updating this section with useful information.