Virus-like program that spreads automatically to other computers by sending itself out by email or by any other means. A program that propagates itself by attacking other machines and copying itself to the affected machine.
Worms have self-replicating code that travels from machine to machine by various means. A worms first objective is merely propagation. Worms can be destructive depending on what payload they have been given. Worms may replace files, but do not insert themselves into files.
Win32.Wisria is a worm that spreads to contacts stored in Skype Instant Messenger. This is done by sending skype chat message containing a malicious link in it to all contacts in Skype.
Once the link is clicked, a malware file is downloaded and executed which in turn downloads additional files and spreads further.
This worm has the ability to check whether or not it is being debugged, so that it can modify its behavior.It also targets some security applications.
More information about this threat can be found at http://blog.spywareguide.com/2007/05/new_skype_worm_variant.html