It occurred to me today that bitcoin is not democratic.  The direction of the blockchain is determined by majority yes, but certain people hold more of it than others.   As I understand it, the biggest mining pools essentially determine which client to follow, and we trust them to always do what makes the most financial sense to them. 

As bitcoin's population we ride a delicate balance that is ripe with the seeds of conflict.  For now our attention is focused on bitcoin vs. regulation, but I have no doubt that someday internal changes will cause various bitcoin subgroups to be pitted against each other: either between big holders and little holders, or miners vs. traders, or devs vs. users.  One dev might introduce a change that is deemed profitable by some, and disastrous to others. A new client is introduced, some insist on a boycott, and then we have a problem.  I've been trying to research what happens with a true hard fork but any searchers are saturated with the recent news of the may 15th update.

Can someone actually explain what happens with a true hard fork? 
Will conflicting clients create persistently separate blockchains? 
Will someone's bitcoins exist within each blockchain? 
What major things could we expect?