A Dynamic Multipoint VPN is an evolved iteration of hub and spoke tunneling (note that DMVPN itself is not a protocol, but merely a design concept). A generic hub and spoke topology implements static tunnels (using GRE or IPsec, typically) between a centrally located hub router and its spokes, which generally attach branch offices. Each new spoke requires additional configuration on the hub router, and traffic between spokes must be detoured through the hub to exit one tunnel and enter another. While this may be an acceptable solution on a small scale, it easily grows unwieldy as spokes multiply in number.
DMVPN offers an elegant solution to this problem: multipoint GRE tunneling. Recall that a GRE tunnel encapsulates IP packets with a GRE header and a new IP header for transport across an untrusted network. Point-to-point GRE tunnels have exactly two endpoints, and each tunnel on a router requires a separate virtual interface with its own independent configuration. Conversely, a multipoint GRE tunnel allows for more than two endpoints, and is treated as a non-broadcast multiaccess (NBMA) network.