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).

Reserved characters (such as < and >) that should not be interpreted as HTML should be represented in the data as HTML entities (such as &lt; and &gt;) or replaced with HTML entities after extraction using a Post function (in the Extract step properties).

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