Muggles' Guide to Harry Potter/Magic/Portus

Overview
A Portus spell is used to create a Portkey.

Extended Description
A Portkey is a magical means of transportation between two specific points, potentially at a specific time. The spell Portus is used to create a portkey; the visible effect is that the object being charmed glows blue and vibrates briefly.

Professor Dumbledore creates a portkey on two occasions in Harry Potter and the Order of the Phoenix: when Mr. Weasley is injured, he creates a Portkey to carry the Weasley children and Harry to Number 12 Grimmauld Place so that they don't have to deal with Professor Umbridge; and after the battle at the Ministry, he creates one to carry Harry back to his office. In this latter case, Cornelius Fudge, the Minister for Magic, is present, and is scandalized that Dumbledore would dare to produce an unauthorized Portkey.

Analysis
The Portus spell must be complex indeed, quite apart from the magic involved. We can safely assume that, like the Riddikulus spell and the Patronus charm, a significant part of the spell is actually derived from what you are thinking of at the time the spell is cast. There are a significant number of things that you must think of when casting Portus. Not only must you hold in your mind the destination that you desire, you must also decide whether the Portkey is to be a there-and-return key, in which case you must presumably also hold the return destination in mind; additionally you must set the time when the portkey is to activate, or whether it is to activate upon command or upon contact. It may be possible to make a personal Portkey as well, in which case you must know whose contact will activate it.