Using DDR, a connection between sites is only established when a specific type of traffic initiates the call or when you a backup link is needed for redundancy or load sharing.

DDR is used in order to save on the costs of a dedicated WAN line for organizations that do not need permanent continuous connection and as a back-up by organizations that use the dedicated line for critical applications.

(2) See DDR-SDRAM.

Dial-on-Demand Routing  
Detailed information from Cisco about DDR.

Understanding Demand-Dial Routing
While the concept of demand-dial routing is fairly simple, configuration of demand-dial routing is relatively complex.