A hoax in the context of IT security is a warning of a virus or threat, which will allegedly have the most terrible effects on the affected system. This goes as far as the suggestion that the system hardware will be destroyed or set on fire.
In most cases there is next to no truth in such horror stories. Nearly all hoaxes are based on the fact that the recipient of the mail will forward it to as many other contacts as possible, so that it snowballs and acts like a classical chain letter.
The topics vary widely and extend from the killer viruses already mentioned, through the claim that software manufacturers will reward you with a cash prize, if the corresponding mail is sent to enough recipients, to apparent petitions with reference to highly relevant topics, such as halving petrol prices.
With a little intuition, such hoaxes are easily recognised for what they are. In most cases, there will be terrible consequences should the recipient not act according to the instructions provided in the email. If he observes the instructions, then he will be rewarded with a present or money. With reference to the security of his own computer system, it is often claimed that the malware to which the warning refers, cannot be detected by any antivirus software in the world. If this were the case, how could anybody know of the existence of this purported virus in the first case? The end of the message almost always concludes with the demand to forward the email to as many friends, colleagues and acquaintances as possible. A look at the structure of the mail will also provide evidence that it has already been forwarded many times.
A list of popular hoaxes can be found on the hoax information site of the Berlin Technical University: