Prices are set on exchanges, where a buy order and a sell order have an equal bid price. If Bob puts in an order to sell at $10 each, and Nancy puts in an order to buy at $10 each, then the exchange makes the trade happen, and the market price is then listed as $10.
Now, on different exchanges you have different orders from different buyers and sellers. Thus, the market price on one exchange will always be different than other exchanges. However, the prices will tend to be very similar, because if one becomes much higher than another exchanges, people buy at the lower exchange and sell on the higher one, bringing the prices into alignment. The more efficient a market is, the less difference there will be between exchanges.
Make sense?