Blizzard explanation as well: http://eu.battle.net/wow/en/forum/topic/12822112588
Normal, Heroic and Mythic all share separate lockouts.
Normal/Heroic can be run as many times as you like. You can use bonus roll every time. You can loot each boss on every difficulty one time.
Mythic can be ran just one time and you will not get a personal lock, but instance one.
Normal and Heroic
Those are loot based - like LFR. It means that if you killed all the bosses or just some of them, you still get to run it again as many times as you like. You are ineligible however to receive loot from bosses which you are killed. However you can use your bonus roll as many times as you like. Bonus roll doesn't care if you have already looted the bosses before or not.
The raid leaders lockout is the only thing which matters. If you have killed all the bosses and decide to make a new group with the character you killed the bosses on - your group will end up at the last boss being dead. However if you join some other raid leaders raid, while you had a full clear, you will start from the boss where that other raid leader is at.
One thing to note is that if you killed bosses 1-5, skipped 6-7 and then killed 8-10 and you decide to make a new group as the raid leader to kill the last bosses from 11-14. You will have to kill your skipped bosses (6-7), everyone leaves the instance after that, and then you can re-enter and you'll find yourself on boss 11. As a raid leader the game checks all of your bosses killed, so if you miss couple of earlier bosses in order to be able to get right in front of the later ones, you'll have to kill the missing bosses.
Mythic = old heroic lockout system.
You can loot/use bonus roll only once per boss.
The lockout which you get is instance based. It means - you cannot join anyone else Mythic run even if they have killed more bosses then you. You can form your own group with the lockout if you like to, but other people with their Mythic lockouts from different groups cannot join yours.
Instance based lockout also means that if you kill 8 bosses in one group and you are told that they will continue in couple of days from the next boss. You should not go and form the group of your own with the lock you got. If you form your own group, end up clearing the place, you'll mess everyone's lock up who were in the initial run. They get saved to the amount of bosses which you killed on your own. Their raid info might show that 8/14, but they are still saved to your amount of bosses killed.
If you run Mythic you cannot join any other raid leaders raid with different lockout nor should you create the group of your own if you are told that people will continue on another day.
All correction of any sort are welcome and will be added.