A DNS zone is a portion of the global DNS namespace. This namespace is defined by the DNS protocol (RFC xxxx) and is laid out in a tree structure from right to left, such that divisions of the namespace are performed by prepending a series of characters bollowed by period ('.'), to the upper namespace (e.g. 'com.' to 'originet-solutions.com.').

The correlation between domain names and DNS zones is that each period indicates a break in DNS namespace where authority "can" be delegated. This delegation may be bureaucratic (as in top level domains which are tracked by organisations and governments) or technical (where a system becomes responsible for the management of information within that zone) or both.

Source: wikipedia.org