Malware is malicious code that a hacker designs to hide in a remote computer. Once infected the computer becomes part of a network, called a botnet, which the hacker can control remotely. Zombie computers will continue to infect more and more computers until the hacker has amassed an army. In 2005, Dutch police foiled a major botnet operation that controlled over 1.5 million computers!
Once under the hacker's control, zombies can be used to do all sorts of naughty things, like spamming and clogging. Spamming is when email address are slammed with tons of unsolicited email. It is estimated that 50-80% of all spam is distributed by computers on botnets. Clogging occurs when a hacker uses his botnet to flood the bandwidth of one or more web servers. In 2000, Yahoo and eBay were clogged to a standstill because of an attack by a 13-year old hacker in Kansas.
Don't let your computer become on of the walking dead. Use antivirus software!