Cisco Express Forwarding (CEF) is advanced Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications, or interactive sessions.

Although you can use CEF in any part of a network, it is designed for high-performance, highly resilient Layer 3 IP backbone switching.

While this is a good BuzzWordCompliant? answer, what does it actually mean? My understanding of CEF was that all the routing was done on the line card (not on the host CPU), so some advantaced features weren't available with CEF.