HtmlUnit is a "GUI-Less browser for Java programs". It models HTML documents and provides an API that allows you to invoke pages, fill out forms, click links, etc... just like you do in your "normal" browser.
Thats what the Parser says.
Apart from this, most parsers will tokenize html code into units, so you would have to deal with tokens instead of plain text and have some semantic information about the html code you are dealing with.
Depending on the parser, the tokens may be different.
Some tokens may tokenize the data into objects and array(which i think HTML
unit does).
Mostly all will parse it into xml.