A domain name is a unique name that is used to identify a website/computer on the internet. A domain name is a series of words (usually memorable) separated by dots (e.g. idaq.com) which is used to find an IP address. Computers need to know each others IP addresses in order to communicate over the internet. The DNS (Domain Name System) enables a computer to lookup another computers IP address when given a domain name.
Domain names are always composed of at least two or more parts separated by a dot. In terms of addressing, the left most part of a domain name is most specific and the right most part is least specific.
For example, in the address www.idaq.com.