Domain Name System
As we all know in a network, the computers communicate with each other using IP address. For humans it’s hard to remember the IP address of each computer. To make things simpler, DNS was introduced. The main role of DNS is to converts the Domain Name to corresponding IP address. Hence the Domain Name points to the server.
DNS is static which means it remains fixed for a particular domain.
Dynamic Domain Name System
DNS is static so the IP address associated with it should be static. If you have a dynamic IP, the computer changes the IP address frequently. So by accessing the Domain Name, you will not be able to reach the desired computer. Dynamic DNS is used to in order to avoid this situation.
In cases if a website is hosted on a server with dynamic IP, each time you connect to the internet, a temporary IP address is assigned to the server by the Internet Service Provider. So when the IP address of the server changes you will not be able to access the website using Domain name. In such cases the Dynamic DNS will take care of this issue by changing the IP address of your website correspondingly.
http://www.techcuriosity.com/tutorials/networking/difference_between_DNS_and_DDNS.php
https://www.quora.com/What-are-the-differences-between-DDNS-and-DNS-and-how-are-they-used