HTMLString

HTMLStrings contain textual data that includes HTML markup. They are essentially the same as String values except in cases where the HTML markup can be interpreted.

Example: Assume that a field has the value He said <b>WOW!</b>. If the data type is String and the value is placed in a template, it will display exactly as "He said <b>WOW!</b>" (without the quotes). If the data type is HTMLString, it will display as "He said WOW!" (again, without the quotes).

Considering this is the only difference, for more information on how to create and use HTML String values, see String.