Monero is the leading cryptocurrency with a focus on private and censorship-resistant, transactions. Almost all cryptocurrencies (including Bitcoin and Ethereum) are transparent. They reveal your entire transaction history to the world. Monero uses unbreakable cryptography to ensure that no one can see how much money you have or where you spend it, even if they know your wallet address.
Monero by design introduces the notion of one-time random addresses. The idea is that each sender of a transaction generates a new one-time random address for the recipient in a way that only the recipient can spend it using a long-term secret key. If each address is generated using fresh randomness and is used only once, then it should be hard for an adversary to link two addresses. Monero enforces untraceability using a cryptographic primitive called ring signatures. The primitive allows a sender (also the signer) to anonymously sign the transaction (the message) on behalf of a set of other users. As a result, the real output being redeemed remains anonymous amongst the chosen set of outputs belonging to other users. The guarantee that Monero aims to achieve is the privacy à la anonymity-set size.