Cryptocurrency exchanges serve as intermediaries where users can buy, sell, or exchange cryptocurrencies for other digital currency or traditional currency like US dollars or Euros. They can be centralized (controlled by a single entity) or decentralized (operating on a peer-to-peer basis), and they vary in terms of security, privacy, fees, and the number of cryptocurrencies they support. Transactions and trades on exchanges are executed using trading pairs, which consist of two different currencies that can be exchanged against each other.