CEF (Cisco Express Forwarding) is a technique to increase the packet switching speed. When a router has to forward an IP packet, it has to perform a lot of different tasks. For example, it has to do a lookup in the routing table to find the destination, it has to find the destination MAC address of the next hop, recalculate the TTL and check if there is an outgoing access-list that prevents forwarding the packet. In a nutshell, CEF combines all this information in a table so that it can quickly do a lookup instead of having to check all these different sources.