In order to purchase, sell, or trade cryptoassets, you must utilize an exchange. Exchanges come in two forms: Centralized and Decentralized.
Centralized exchanges are websites where traders can go deposit their cryptoassets and engage in the marketplace. The site itself is responsible for the security of the assets and maintaining their own liquidity pool.
Decentralized exchanges are websites that act as marketplaces for traders to exchange their cryptoassets without ever depositing them onto a centralized exchange. They are considered more secure as there is no central location (target) for hackers. The problem lies in liquidity.