A Google virus is a software virus that in some way involves the Google corporation.
Of course, the Google corporation has no intent to launch software viruses on the world. A Google virus is created by a malicious third party. A Google virus may be used to attack Google and/or is inadvertently spread by someone in the Google corporation (we can all spread viruses by accident, it's just when the brainiacs at Google do it we think, rightly, 'it can happen to anyone', and start to worry).
A virus, in general, is a software program that can reproduce itself and is often designed to cause great harm to files or other programs on your computer. Viruses, including Google viruses, are everywhere, so the best investment you can make, after buying a computer, is to purchase a good anti-virus package.
Note that a true computer virus cannot spread to another computer without human assistance; the most devastating type of virus capable of doing this is more accurately described as a worm.
The term Google Worm has also been applied to an unethical person who has had some, direct or indirect, dealings with Google (for reasons I go into elsewhere).
One famous Google Virus, a true virus, was the one that Google themselves inadvertently distributed as an email attachment to the google media list in August 2003.
So don't trust email from anyone and make sure you have an anti-virus application installed.