Dynamic DNS (DDNS) allows a client machine or network device to automatically "register" in a DNS domain (rather than the traditional static entries usually used in mapping DNS to servers).
This is particularly useful when the client machine's or network device's IP address is assigned by a DHCP for that subnet, as it will allow the client's machine hostname to be mapped to the current IP address dynamically.
A similar function for mapping host/device IP address to NetBIOS name was also done Microsoft Windows NT system via WINS in a flat domain structure. WINS is rare nowadays, as NetBIOS is not as commonly enabled. Most IP networks use the DDNS over a straight IP network to do this function.
Dynamic DNS is a method, protocol, or network service that provides the capability to a networked device using the IP protocol suite, such an IP router, or computer system to notify a domain name server to change, in real time (ad-hoc), the active DNS configuration of its configured hostnames, addresses or other information stored in DNS.