summary: "a group of cryptographic signatures with at least one real participant, but no way to tell which in the group is the real one as they all appear valid"
In cryptography, a ring signature is a type of digital signature that can be performed by any member of a group of users that each have keys. Therefore, a message signed with a ring signature is endorsed by someone in a particular group of people. One of the security properties of a ring signature is that it should be computationally infeasible to determine *which* of the group members' keys was used to produce the signature.
For instance, a ring signature could be used to provide an anonymous signature from "a high-ranking White House official", without revealing which official signed the message. Ring signatures are right for this application because the anonymity of a ring signature cannot be revoked, and because the group for a ring signature can be improvised (requires no prior setup).
A ring signature makes use of your @account keys and a number of public keys (also known as outputs) pulled from the @blockchain using a triangular distribution method. Over the course of time, past outputs could be used multiple times to form possible signer participants. In a "ring" of possible signers, all ring members are equal and valid. There is no way an outside observer can tell which of the possible signers in a signature group belongs to your @account. So, ring signatures ensure that transaction outputs are untraceable. Moreover, there are no @fungibility issues with Monero given that every transaction output has plausible deniability (e.g. the network can not tell which outputs are spent or unspent).
