http://www.techradar.com/news/the-best-bitcoin-exchanges-2017

A Bitcoin exchange is a digital marketplace for users to buy and sell Bitcoins using different currencies. The Bitcoin can be exchanged for either fiat money (legal tender) or other alternative cryptocurrencies such as Ethereum. The exchange serves as the middleman for traders of the cryptocurrency.

Being able to purchase Bitcoin is the initial step to trading in it. Just like trading a stock, Bitcoin exchanges charge transaction fees, which range from 0 to 1%. In order to trade Bitcoin on an exchange, the user needs to have an account, and go through a series of steps to be verified. Remember that Bitcoin and alternative cryptocurrencies are a completely digital currency, and there are no physical Bitcoins.