A trojan is a program that acts like the Trojan horse of ancient legend. It is said the people drew this "offering" from the gods into their city to celebrate and when they set off to sleep, the soldiers inside opened it up, opened the gates and the war was lost. That's what happens with a trojan program, it looks to your system like a normal Windows program, say an editor, and what it is really doing is changing code on the boot blocks of your disk or doing other nasties. A virus is a much like a trojan except that it is "injected" into an already exiting program and the same thing happens.
The bot is perhaps the most insidious kind of program as it literally goes through all of your mailing lists, contact lists, Facebook lists and any other address lists and spams them with all kinds of spam or establishes backlinks through which third parties can spam the world and it looks like it came from your machine.
AVG Anti Virus puts a stop to that cold. Using some really cool programming techniques and things such as heuristics, where certain rules are set up by AVG and changed as needed by either you or the program - you computer "thinks" for itself, and it puts the trojan right back into its horse and then rides the horse into the computer ground and the same thing with trojans and bots. Suddenly, your machine is no longer the highway to spam that it had been.
More than that, it looks for rootkits, or software that actually changes the kernel software on your machine so that it does can turn the system that is "working fine" today into tomorrow's deep headache as it affects your department or company network with issues where software stops working correctly and even your IT experts can't figure it out.
One injection with AVG and rootkits are history as are those nasty trojans, viruses and bots. And, in an another nasty twist some particularly nasty folks out there have found ways to watch your Web use and from it insert software that looks like it has come from your system into the Web. Known as Link kits, they make your computer part of networks that you may not want to be part of so AVG watches for them, roots them out and destroys them.
The best thing about AVG is that it all happens in the background. You receive upgrades to the software in the background so that a kit which appears at 2 pm may be gone by 8 pm as some developer on the AVG team has found a way to zap it into cyberpsace.
For the most part you'll like what you find and its all just a download away at AVG Anti-Virus 2012. It is an important piece of software that could use a little better firewalling but knowing AVG it is probably coming, too.