DMARC is an email standard promoted in January 2012 to prevent email fishing, spam and other abuses. DMARC stands for Domain-based Message Authentication, Reporting and Conformance and is notably supported by AOL Inc., Google Inc., Microsoft Corp., Return Path and Yahoo Inc.
DMARC is designed to fit into DKIM and SPF email authentication processes and to give senders feedback when theirs domains are used as signature for emails.
With DMARC the legitimate domain owner can signal to receivers that he is using email authentication (SPF, DKIM) and provide an email address to gather feedback about messages using his domain - legitimate or not. He can also specify a policy (report, quarantine, reject) to apply to messages that fail authentication and seem to originate from his domain.
The DMARC process (image credit DMARC.org)