If your PC is a bot or has been attached to a bot network, it means that you have been infected with malware that puts your machine under the control of a master machine somewhere in the world. There are claims that some bot-nets are composed of millions of PCs.
There are two main uses for a bot net. The first use is as a spam-bot. This is a member of a bot net that has been instructed to send out emails, typically advertising Viagra or Cialis. They will use stolen email lists and I’ve had customers ask me to clean their computers of viruses when their contacts ask them why they’re sending this stuff. Usually this happens with gmail, AOL, and other on-line accounts where the user hasn’t concocted a strong password. Their on-line account was hacked, not their PC.
Another use is to execute a cyberattack on some site, group or organization that has incurred the bot net owner’s wrath. Alternatively the bot-net owner might be being paid by someone to do perform the attack. The most common cyberattack os the DDOS, Distributed Denial of Service, attack. In this many bot computers send requests to open a connection to the target. The target duly posts this request, a process that uses up a slot in the connection request table and sets a fairly generous timeout, and sends the appropriate response. The original connect request was hacked such that the response goes off to never-never land. The bots continue to send requests. With a large number of computers sending these bogus requests and then letting them slide the target computer’s available number of connection slots gets rapidly used
up. As the timeouts expire the slot is released only to be snatched up by the next bot’s request. As you can see this attack can bring a server to its knees.