If I were to choose something to describe Anti-Virus software, it would be a bank that is funded by the mob. No, seriously. A bank, that you put your money in, operated by the mob. Think about it. Is the analogy not sticking yet? Let me explain.
You computer is a pile of cash, but if you leave it unattended, it will attract people wanting to take your money. So in order to protect it, you need to put it in a bank. So you take your cash to a bank, but realize there are many kinds of banks around. Now, there are three options from here on out, as I can explain in detail.
1) You put all your money in one bank. This seems the most practical, but not all banks are created equal. Some will give you interest, some are state of the art, and some will charge you an annual fee to protect your files, much like the mob. You wouldn't want anything to happen to your files, right? We would hate for a...say....worm to get ahold of your money, right? So do the right thing and pay us $50-$70 annually in order to insure your money. Also, we will block anything that remotely looks like a virus. We put your money in a plastic bubble and have guards on it 24/7/365/etc. NOTHING will get through to your stuff, including harmless email from your friends or a site that you know is legitimate.
2) You diversify and put your money in several different banks. Safety in numbers, right? Wrong. You are asking mob members from the north side of town and the mob from the south side of town, and the mob from Italy, and the Triad to guard your money. Each will require its own payment, and each has its quirks. On top of that, the mobs won't play nice with each other. This is an important fact to remember. You will probably not get access to your money because the mobs from the north and Italy are having a dance off to determine whether Internet access should be granted.
3) You do nothing and the mobs steal it all anyways.
However, you don't need a mob to protect your pile of money. What you need is a nice big 'ol hole in the ground. This is also known as freeware. Freeware is like the mob, only they won't make you pay for their services. Again, its not a good idea to dig several holes and put your money in those as you will forget where they are and will never see the cash again, but choose a good reputable hole to stick your money in.
So yes, that is how I would describe Anti-Virus Software. Its necessary to keep your files from being compromised, but for the love of Pete, just go with the freeware as they provide the same kind of protection but without costing you money. As of this writing, I suggest either Microsoft Security Essentials or Avast in combination with Spybot Search and Destroy. They have kept my computer safe for years.
Also, browse with Firefox instead of Internet Explorer as most viruses, worms, and trojans are contacted through there. I use Firefox in tandem with FeedDemon, but RSS is for another time.
So I hope you have learned something, and maybe understand why I have a hard time explaining things on a lower level. Remember, the mob is only out for your money. Sure, they will protect you, but its like paying off a bully. You're better off avoiding them in the first place and just get something as effective for free.