Privacy in this public permissionless network has been limited. Since the Metropolis hard fork, it became possible to integrate more cryptographic operations in smart contracts - two kinds of technologies are implemented: zk-SNARKs and Ring Signatures.
‘Zero-knowledge’ proofs allow one party (the prover) to prove to another (the verifier) that a statement is true, without revealing any information beyond the validity of the statement itself.
Ring Signatures are a cryptographic technology first introduced in 2001. It enables any member of a group of users to perform a digital signature, that can be proven to be made by a member of this group, while it is impossible to determine by which member of the group.
Ethereum Foundation, Ethereum/Metropolis (vByzantium), all