It is a mass-mailing e-mail worm which spoofs the FROM header. As of this writings there are at least 21 strains of it active. The most reported is strain Q. This strain is easily identified by the subject and message.
The message claims to be a delivery failure and includes a link to a supposed PHP page that will allow the person to see the message that failed to deliver. Clicking on the link infects the recipient's machine.
Specific information on this strain can be located here:
Symantec's NetSky.Q information page
To see information on other strains sinply replace the letter "q" in the worm name to the particular strain you wish information on.