What are they? Basically, there are untold numbers of botnets out there on the Internet. Their creator-administrators, or "bot-herders," put together networks of computers (PCs in homes, offices, companies, even governments) that run tasks without their owners knowing it -- often, sending spam, or marshaling denial-of-service attacks, sometimes nastier things. Bot-herders recruit their networks often by way of a trojan horse badware item sent via spam e-mail or some other means. Once a PC is infected, it logs into a central server where it can be administrated, similar in principle to an office network with a shared server.

Why would you care? If your home PC is part of a botnet, then its security has been breached and you're exposed to whatever else the bot-herder might want to do with your machine -- steal your identity, personal data off your machine, etc. Also, you probably don't want your home computer being used by some black-hat operator in Romania to create a massive denial-of-service attack on some hapless bank or ICANN server or something.

The good news is, standard good-security procedures can help you avoid being bot-netted (remember that if you have a broadband connection, security problems can be somewhat more acute):
1. Don't open e-mails from people, companies, and other sources whom you don't know, and for heaven's sake don't click on animations or graphics or files inside those e-mails.
2. Make sure your PC software is up to date. Suffer the pain of going to Microsoft Update. Get the CD version of Service Pack 2 for XP.
3. Download Spybot Search and Destroy for free and consider making a donation.
4. Download AVAST! or some other free anti-virus program.
5. Maybe enable your firewall. Or maybe not.
6. See if your ISP has some security tools and updates to download.