It's just a "door" that is activated by a trigger other than direct activation - i.e you don't have to click something. Typically with a trigger zone, which is activated by the player entering it, but you could use a floor plate or any other method of indirecting the door activation. Look at the various descriptions around of how to set up a trap, the same mechanisms will work for this.
And incidentally, an Oblivion gate acts more like a regular door, because you DO need to click on it. Walking through doesn't activate it, although there is a mod around to make it do so, which may be a good source of "how to". The name "Walking to Oblivion" rings a bell somewhere, but that might just be a thread title.