IP Multimedia Subsystem or IMS is an architecture standardized by 3GPP, 3GPP2 and TISPAN, that is the choice of telcos to implement not only VoIP, but also other multimedia services, such as videcalls, presence, instant messaging, push-to-x, videosharing, on-line address book, etc.
As an user, you can envision IMS as a service similar to what Skype offers today but standard-based, with assured quality of service and fully inter-operable among different telcos.
IMS software clients will run on different devices including handsets, PC/laptops and IPTV Set-top-boxes.
AT&T U-verse Voice is already based in this kind of technology.
But, if Skype is already free, why do we need IMS?
1) To avoid ending in a monoploy situation with Skype. As telcos adopt IMS, users will be able to choose their favorite service provider, and that will not impact what buddies you can talk to. E.g., you can not access Yahoo IM or GTalk users from Skype. Once Verizon, AT&T and Sprint have all IMS, you will choose your preferred operator, say AT&T, and still reach your buddies at Verizon and Sprint.
2) Quality of Service. IMS provides for guaranteed bandwidth. This will enable Hi-Def (1080) video calls, that best-effort Internet can not sustain.
3) Carrier grade service, including Emergency Services, not available from Skype.
4) Access from multiple devices and network access. Coupled with IPTV and Femto cells, as an example, you will be able to receive SMS, IM or calls on your TV screen or on your PC, even if you have been called on you cellular. With Femto cells, your presence info could be automatically updated when you reach home. Other services bundled with IPTV would include video conference from the TV, or having a voice, video or chat session open with friends on TV while watching the Super Bowl.
5) Open architecture, including a SDE (Service Development Environment) for developers to add Applications blended with your IMS Service. Applications such as those available in Facebook, could be made available on the IMS Service, bringing to power of Social Networks and Web 2.0 together with IMS.
Skype is a great service, but the potential is bigger with IMS, mainly because of the competition it would generate among telcos, that would drive more innovative applications and a better service at the end of the day.