So far in the answers in this thread, Winnuke and the Ping of Death have been mentioned. Winnuke sent out-of-band data on port 139, which caused Windows machines to lock up. The ping of death was a very old attack which sent a big malformed Ping packet (ICMP ECHO_REQUEST) to the victim, which would cause the same sort of effect. More recently, twinge appeared; this sent a flurry of invalid ICMP requests to the victim (A Windows machine) which would kill it pretty quickly.

However, these are all specific attacks. The term 'nuker' or 'denial of service attack' just means any attack on a machine which causes it no longer to be able to provide a certain service or set of services. As such, a brick lobbed with sufficient force is a nuker, too; it's just that most of them are small programs that splatter the machine over a network because that's the easiest, most popular form of this sort of attack.

There are hideous numbers of different ways that these nukers have been known to work in the past. Luckily most of them are highly unlikely to work on most machines these days, though - but the latest ones still will, of course.

Well, I could go into more specifics, but there's no particular reason for me to do so, so I won't. If you have any specific questions about this, feel free to ask and I'll try, along with everyone else here I'm sure, to give a helpful answer.

- Freon