Trapdoor is based on PHPWiki. A wiki is a website with an "edit" button: you can edit the site right online, add links or text, make text bold, add lists of information. All of the site's information is stored in a database. Probably you're not seeing this edit-button right now, because you are not logged in.
This wiki is extended with a lot of functionality, things we really needed, to make, ahem, a design site. The trapdoor wiki, besides letting you edit text, lets you upload files as well (images, flash, shockwave, css, zip, ...). It's link system is more advanced: it supports pop-up windows and image buttons. And it integrates kiwi, which is the same as a wiki, but with line drawings.
Trapdoor's content deals with design, code, and ideas.