Most exchanges match the orders its users, the sellers' orders with the buyers' orders if the price is the same.
They differ by reliability, user interface, the value of the fees (exchange fees, withdrawal fees), the identification/KYC policy they have, the API they offer for use the exchange from 3rd party programs (bots)...
Some exchanges allow exchange between crypto currencies (BTC, ETH, ..) and fiat (USD, EUR), some other exchanges allow exchange only between various cryptocurrencies.
The different exchanges offer different exchange pairs. Some altcoins or tokens are available only on certain exchanges; EUR or RUR are available on certain other exchanges...
Get a list of exchanges, make account, play here and there with small amounts of just look around and pick your favorites.