Cryptocurrency exchanges are online platforms where you can buy and sell cryptocurrencies. They act as intermediary between buyers and sellers. That’s why you have to pay a conversion fee.
There are also decentralized exchanges which operates without a central authority that facilitate the transactions.
You can trade 24 hours a day every day of the year, crypto markets are usually always open. There are two kinds of executable operations: market order and limit order.
With the first one, the fastest, you can buy or sell at the best available price in the market. A limit order is a type of order to purchase or sell a certain amount of a crypto at a fixed price or better.