1. Disk Operating System (written: DOS). An operating system that ran on early IBM compatible computers. Dubbed QDOS (Quick and Dirty Operating System) by its original creator, it was subsequently purchased by Bill Gates who then used it to take over the world (point, narf).

Some programmers and IT workers consider DOS the single most widely spread computer virus in human history.

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2. Denial of Service (written: DoS). A method used by hackers to disable a server or set of servers on the Internet by bombarding the target(s) with data traffic. Methods have become more sophisticated over the years to include such things as setting up Trojan horses on multiple servers and sources around the web which are all programmed to send traffic to the target server at the same time. This is called a Distributed Denial of Service attack or DDoS.