Different exchanges have their own internal markets, and the price of Bitcoin in the exchanges depend on the buyers and the sellers in the exchange, so the prices fluctuate a lot. However, users can buy and sell between multiple exchange sites so that they can buy at the lowest and sell at the highest.
Thats why it is an advantage to trade at multiple sites to get these benefits, however, bear in mind the trading fees and the withdrawal fees, so unless you are trading at a large volume, it is still not worth it.

Thank you for the response.
Just to make sure I understand. A person can buy a bitcoin on one exchange for cheap price, then sell on another exchange for higher price?
Why not just buy and sell higher on one exchange? I am not sure I understand?
Thanks
Yes you can buy at the cheapest price at one exchange then transfer to other site where the price is higher but it wont worth enough for small amount of btc because the most possible gap would be just small amount in $. It is not recommended thing unless the price difference is so high. Using one exchange to buy and sell is enough for most traders. Just wait patiently till the price goes up to make profit.