Essentially, a botnet is a collection of 'zombie' computers which have been infected by a worm, virus or Trojan by an operator called a 'bot' herder. Make no mistake, a botnet is never used for the purposes of good.
Clicking on links contained in an unsolicited e-mail is one of the most efficient ways to become part of a botnet.
So what would you see if you clicked on any of the links in any such e-mail?
Most likely nothing at all because the process by which you will be infected has been hidden from view. Possibly a faked '404' error page which lulls you into thinking that nothing happened. Maybe a sub site which gives you even more options to pick up an infection.
What kind of infection will you have 'adopted'?
The possibilities are almost endless, and are down to the imagination of the bot herder. Here are some examples.
Denial of Service (DoS) Attacks
The botnet is used to bring down a network or computer connection, by flooding the connection bandwidth to the point where the system being attacked can no longer cope with the flow. This kind of attack can be used to bring down a website, and is the basis for booting in Yahoo chat rooms.
Spam and Traffic Monitors
The botnet is used to harvest e-mail addresses with a view to expanding spam and phishing mail distribution.
Also, a bot can look for usernames and passwords which the botnet herder can use for whatever purpose, which may include hijacking somebody else's botnet.
Key logging and Identity Theft
Key logging is used to detect words before and/or after the name of your bank, eBay, PayPal, e-mail clients or anything from which the bot herder may gain.
Identity theft is really simple. The computer user is lured to what looks to be an official website where the aforementioned user happily types in whatever personal information being asked: Name, birthday, postal address, password, username, bank account number, all kinds of stuff.
There is no magic in all of this, and the technology has been around for a long time. There has also never been a shortage of people willing to be scammed.
So how can you tell?
.. if your computer is being used as part of a bot-net?
If you find yourself suddenly facing ISP charges for going over your band-width limit and you know that you are not doing any more on the Internet than usual, check out the status of your Network Interface Card.
Look at the Send and Receive numbers. If the receive number rises slightly, you are probably getting the enxt batch of email. That is ok.
If both Send and Receive numbers are rising very quickly and look like they are in a race to the finish, you have just found the cause of the excess band-width issue.
The only way to get out of the mess is to save your important stuff to external media, and re-install Windows.
How to avoid bot-nets..
Employ a good quality, small footprint anti-virus solution and ensure that you keep it updated. If you don't want to have to pay, use Microsoft Security Essentials.
If your anti-virus tells you to stop doing whatever it is that you want to do, STOP doing it. Use common sense.
Keep your operating system updated. Do not go along with those who claim that updates are trouble.