IN PLAIN ENGLISH:
An exchange is a central meeting place (a website or trading platform) where people looking to buy crypto can easily find sellers, and sellers can find buyers. For the convenience of bringing everyone together, and facilitating the transactions, the exchange takes fees and commissions.
The details:
Using an exchange is the easiest and most convenient way to get involved in the cryptocurrency market.
There are many exchanges operating in the cryptocurrency space, and it might feel difficult to decide which to use. So here are some main things to consider:
• How liquid is the market? Market liquidity has to do with how much volume is traded per day. A less popular exchange, with less trading volume may make it more difficult to execute your trades (buy and sell) quickly and at the price you want
• Which country is it in? An exchange hosted in your own country can make transactions easier, and have fewer complications overall
• Which currencies do they trade? Different exchanges trade different cryptocurrencies. Although it's safe to say that all the exchanges will trade the major coins (Bitcoin, Ethereum, Litecoin, etc), you will want to consider which other coins they trade if you are interested in less common altcoins.
• What are the deposit/withdrawal options? Check that there are easy options for you to get your money in and out of the exchange before registering
• How much are their fees? The fee structures at different exchanges may vary, so it's good to check that you understand the fees before trading. Many exchanges will charge a fee upon depositing money, and then a small commission for executing trades.
• How trustworthy are they? There are many large exchanges with a good reputation and a good user base. Others may have a less than stellar reputation..so search online for what others have to say about an exchange before signing up.
I don't endorse or recommend the use of any particular exchanges, however here are some popular ones:
Coinbase
Bittrex
Bitstamp
Binance
Bitfinex
Quadriga
Kraken