An exchange is defined as a place where something of value can be traded. One of the purposes of an exchange is to ensure fair trades are conducted.
Traditionally, stocks were a common item traded on exchanges. In the crypto world, new exchanges appear to buy and sell cryptocurrencies.
In Exchanges users are allowed to buy and sell tokens for other cryptocurrency (for example sell Wibson Tokens and get paid with Bitcoins) and also each day more exchanges allow users to buy or sell cryptocurrencies and pay (or get paid) with Fiat (USD dollars for example).