Spoofing is electronic identity theft. It consists in posing as somebody else in order to send computer viruses or spam.
This is why you sometimes receive an e-mail containing a virus from somebody you know well, while the person who sent it is not necessarily infected.
You may also have received bizarre messages indicating that you have sent a virus to somebody else even though this is not at all true. It is almost certainly be a spoofer who has stolen your e-mail address!
There is also what is known as "Brand spoofing", i.e. stealing the identity of big companies. The most well known example is the false message from Microsoft accompanied by a false security patch which is actually a virus.
Naturally all this happens automatically on a large scale using software robots. The e-mail addresses are collected on the Web or elsewhere, e.g. in the address books of people actually infected with the virus.
The current problem with e-mail is that any address can be used to send messages because the system itself is not secure.