world user forum

I envision a user forum built around a catalog of categories.

let's try modeling it in xml
<worlduserforum>
<category id="computing">
<category id="hardware">
<category id="laptops">
</category>
<category id="desktops">
</category>
<category id="pocket">
</category>
<category id="tablet">
</category>
<category id="mainframe">
</category>
<category id="super">
</category>
<category id="parallel">
</category>
<category id="network">
</category>
</category>
</category>
</worlduserforum>

The programming objective is to build a database of this type by collecting user input using forms.