IMS (IP Multimedia Subsystem) is an emerging architectural framework, based on SIP and IP, for offering multimedia and VoIP services, as well as fixed mobile convergence. IMS has been standardized by the 3rd Generation Partnership Project (3GPP). The 3GPP specification covers all GSM wireless services.
IMS is being adopted by both wireless and wireline providers as a mechanism for enabling "anytime, anywhere" applications. IMS provides support for the delivery of multimedia applications and voice service over IP transport. The IP transport is the key in that it allows IMS to communicate over any network that supports IP.
Think of how you communicate today. Your voice calls travel over a traditional voice network, a cellular network or a data network enabled for VoIP. The same goes for data, because it can be delivered over both data and cellular networks. IMS provides a common framework and protocols that define how IP networks handle voice calls and data sessions. IMS is an overlay solution that sits above the underlying networks to provide true interoperability among IP enabled devices.