Dynamic Multipoint Virtual Private Network (DMVPN) is a dynamic tunneling form of a virtual private network (VPN) supported on Cisco IOS-based routers and Unix-like Operating Systems based on the standard protocols, GRE, NHRP and Ipsec. This DMVPN provides the capability for creating a dynamic-mesh VPN network without having to pre-configure (static) all possible tunnel end-point peers, including Ipsec (Internet Protocol Security) and ISAKMP (Internet Security Association and Key Management Protocol) peers. DMVPN is initially configured to build out a hub-and-spoke network by statically configuring the hubs (VPN headends) on the spokes, no change in the configuration on the hub is required to accept new spokes. Using this initial hub-and-spoke network, tunnels between spokes can be dynamically built on demand (dynamic-mesh) without additional configuration on the hubs or spokes. This dynamic-mesh capability alleviates the need for any load on the hub to route data between the spoke networks.
DMVPN is combination of the following technologies:
Multipoint GRE (mGRE)
Next-Hop Resolution Protocol (NHRP)
Dynamic Routing Protocol (EIGRP, RIP, OSPF, BGP)
Dynamic Ipsec encryption
Cisco Express Forwarding (CEF)
Reference: http://en.wikipedia.org/wiki/Dynamic_Multipoint_Virtual_Private_Network