Network connections: Any wannabe "hacker" that wants to spy on you or severely mess up your computer. Usually binds to port 12345.
Backdoors: Invisibly installs a NetBus server on your system, which can be used to gain unauthorized access to your computer.
HD Snooping: Whatever your wannabe "hacker" wants to snoop/corrupt/delete, including your sensitive documents and email.

These programs allow people to connect to your computer and spy on what you're doing, format your hard drive, read your email, spoof your IP address, etc. NetBus in particular has a slew of nasty features including the ability to remotely read/type any text into any window, steal passwords, close windows/programs you have running, sieze control of your mouse, open/close your CD-ROM, run programs (like format.com), delete files and take copies of your screen. Remote losers even can (and do!) pop up windows advertising porno sites and the like (so in a sense this falls under the category of "Adware" as well). Likely, the first indication you'll have of a Netbus infection is a foreign window popping up in your face telling you to go to someone's porn site!

You "catch" one of these trojans when you download program files which contain them (notoriously from warez sites), or receive similar infected programs as email attachments. When you run the program, it dumps a file in c:\windows\system or someplace similar, and puts a reference to this file in your Registry so that it runs everytime you start your computer (similar to putting it in your Startup folder, but much harder to find).

Solutions:
Many newer antivirus programs will recognise and remove these programs from your computer. Failing this, you might get some warning signs (see above) before anything serious happens to your machine. If you do, snag some trialware antivirus software as soon as possible! You don't have to pay for it or anything, just run it to get rid of your infection. And then, practice safer cybersex!
 

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