So, what is a stock exchange?
Let's take a look at Wikipedia. Stock Exchange is an organizer of trade in commodities, currencies, securities, derivatives and other market instruments.
In simpler words, an exchange is the same market you usually go to, for example, to buy apples.
When you come to the market, what do you see? A lot of stalls offering the same product, apples, but each seller will have a different price. To illustrate this, look at the picture.