<!ELEMENT elementName contentType >
> Content Type can be:
1. EMPTY indicating that the element has no content.
2. ANY meaning that any content is allowed
(Elements nested within this still need their own declarations).
3. Mixed content where the element contains text, and possibly also child elements.
4. A child declaration using a regular expression.
<!ELEMENT elementName ( nestedElement1, nestedElement2＋ ) >
> Means that an element with a given name will have a nestedElement1 followed by 1 or more 'nestedElement2'
<!ELEMENT Country ( Name, Population, State∗ ) >
> Means that a country element will have a name element nested, a population element, and 0 or more State elements.
<!ELEMENT book ( #PCDATA ) >
> Means that the element will have plain text inside