HMAC (Hash-based Message Authentication Code) is a method constructing a message authentication code based on a cryptographic hash function. The HMAC construction was published in 1996 by Mihir Bellare, Ran Canetti and Hugo Krawczyk. It is standardized in RFC 2104.
HMAC can be proven to be secure as long as the hash function satisfies some rather mild security assumptions. In the original 1996 paper the security of HMAC was proven based on the assumption that the hash is a "weakly collision resistant" iterated hash function and that its compression function is a pseudo-random function (PRF).
In 2006 Mihir Bellare published a new security proof of HMAC based solely on the assumption that the compression function is a PRF or, alternatively, that the hash is "computationally almost universal" and that the compression function is a privacy-preserving MAC. In particular, the new proof shows that, despite the practical collision attacks known against the MD5 hash function, the HMAC-MD5 construction remains secure (at least as long as no new attacks are discovered).