Strategic Rocket Forces conducted a successful test launch of an intercontinental ballistic missile, which was declared a missile of a new type, designated RS-24. The launch was conducted at 14:20 MSK from the Plesetsk test site toward the Kura test site in Kamchatka. The missile was launched from a mobile launcher and carried multiple independently targeted warheads.
The missile appears to be a version of the Topol-M missile modified to ensure compliance with the START Treaty. The treaty prohibits increasing the number of warheads on missiles of existing types. No details of the modification are available at the moment.
UPDATE: The grainy TV footage of the launch indicates that the missile indeed looks very much like Topol-M. It is, in fact, possible that the only thing that distinguish RS-24 and Topol-M is the designation. And, of course, the fact that the "new" missile carries multiple warheads. Topol-M appears to be capable of carrying MIRVs without any modifications. The name change may be a cosmetic measure that allows to avoid a direct conflict with the letter of the START Treaty. Of course, other explanations may emerge when (and if) more information is released, but I wouldn't be surprised if RS-24 is indeed a MIRVed but not otherwise modified Topol-M.