Anyone can post any question they like.
To encourage people to answer it, they can pay a bounty that will go to whoever gives the final answer.
Anyone can answer the question.
To encourage them to answer accurately, they can post a bond. If somebody else's answer is accepted, their bond will go to that person instead.
Anyone can change the answer, but they have to double the bond.
If there is a disagreement about the answer, the people who disagree can keep doubling their bonds for as long as they like.
You can take over an answer that someone else has posted, but you have to split your winnings with them.
If you post an answer that someone has already posted, an amount equal to their bond will be deducted from your winnings.
The result is settled automatically.
Once nobody has answered for a while, the question is settled on the final answer. The time it takes until a question is settled is decided when the question is first asked.
You can pay an arbitrator at any time before settlement.
Paying an arbitrator is usually a last resort, after someone insists on posting the wrong answer, and paying for it with a bond.
If you raise the bond far enough on the right answer, you'll be able to pay the arbitrator and still make a profit.
The arbitrator can be any Ethereum contract.
The arbitrator contract may be controlled by a company like Reality Keys, or a group of people, DAO, or any other decision-making process you can code.
For more detail, see our documentation.