[Mediaeval]

Could someone explain what a DoS is? I'm not familiar with the term.

[Cyber Liberty]

Denial of Service.

[egarvue]

Denial of Service attack. In the simplest of terms, basically, an attacker, using hundreds of machines he has hacked or infected, starts having those machines send out a constant stream of requests for the target server, hoping to overwhelm it and cause it to crash or choke up. It would be like having you and a thousand of your best friends all try to get to Foxnews.com at once. And then keep trying.

[Freemeorkillme]

The practice of sending an overwhelming amount of packets of data to a particular location which prevents the target machine from being able to send out any packets of data of its own, denying service of the target machine, effectively "knocking" the target machine "off-line". Quite an easy thing to do, but takes a little know how to cover up the data footprints.

[strela]

Could someone explain what a DoS is?

There are actually two types of these attacks, DoS (Denial of Service) and DDoS (Distributed Denial of Service). DoS attacks originate from one source while DDoS attacks originate from multiple locations.

The examples of DoS and DDoS provided by others in this thread are pretty much valid. It is possible to use other people's computers as launching points for DoS and DDoS attacks without them ever knowing it.