Virus, Trojan, Worm
Basically, they all fall under the generally category of "viruses". However, there are a few distinctions.
Virus - Technically, a virus infects another file (attaches or inserts itself into it). They usually infect program files or MS Office documents. From there, it can replicate, do damage, etc. Unlike a worm, these do not function as a stand alone (except possibly to infect a given file).
Worm - This is almost identical to a "true virus", except that it lives on its own and generally doesn't infect other files (although it can replace them). Usually, these copy themselves using e-mail, networks, disks, etc. Again, these are very close to a true virus, and can do the same kind of damage.
Trojan Horse - This type of program doesn't copy itself but does do damage to your computer. These types of programs rely on people to pass them around and to run them. They do not e-mail themselves. The idea is to make the program look like it's something harmless, like a screen saver or joke, so it gets sent around.
Web Bugs
Have you ever heard the term Web Bug? How about Web Beacon or Clear GIF? Well, don't feel left out if you haven't, it's not everyday terminology. The terms I mentioned are different names for the same thing. For the sake of remaining simple I'll stick with one name Web Bug. So what are Web Bugs and what to they do? They are small (1 by 1 pixel), usually transparent gifs hidden on websites or e-mails within other images like banner adds. Web bugs come from a different site than the one you are viewing but are most often affiliated with the site being viewed.
They are a lot like cookies in that they allow sites to recognize you and track where you've been. Web Bugs have a couple of major differences however. Web bugs share your information with any site that you may visit with the same type of web bug imbedded on it. The personal information bugs are sharing can be harmless information or very personal information, it depends on what information you have put on one of these sites. If you have given one of these sites information like your email address, then every other site you visit with the same bug will have your email address and can now send you spam. By sharing this personal information between sites these entities using bugs can get your email address and send out more spam and web bugs right to your front door. If you visit a site that recognizes you from a Web Bug then that server can not only track you but also send images and blocks of text to you. Web Bugs are also harder to detect than cookies because they are hidden, extremely small, and invisible.
These Web Bugs can also be placed in e-mails, a favorite trick of spammers is to hide a Web Bug in a random email. Opening or even previewing this e-mail can initiate the Web Bug process, which tells spammers that this is a valid e-mail address when someone views it. This validates your address, essentially making you part of their list of e-mails to spam. Most of the more recognized e-mail clients are affected by this such as Outlook Express, Gecko for Linux, Netscape, AOL, and more.
So, how do you stop or prevent these web Bugs from tracking you? As far as e-mail, make sure you don't open or even preview e-mails from unknown sources. Another thing you can do to prevent the Web bugs from biting is to block images in your e-mail all together, this will stop the GIFs from getting through your defenses. Win XP Service Pack 2 turns this setting ON by default in Outlook Express, and other e-mail clients have similar options (i.e. MSN/Hotmail and AOL).
You can also configure tougher cookie settings in your browser, but keep in mind all of these settings will affect the way you surf the web and view your e-mail. You can get advertising blocking software, but these programs have a hard time distinguishing between images that show information and images that are crawling' with Bugs. You can also go through the HTML code and look for the img tag that relates to a cookie (sounds like fun).
So are these bugs really something to get all worked up about? Well if you really don't like the thought of constantly being tracked and evaluated. If you don't like spam or pop-ups then this could be something you might want to at least regulate. You can figure out exactly what a web bug is doing by viewing the privacy policy which should be located on the website of the owner of the bug. If you don't find the privacy policy, you can always send the company e-mail and ask for what purposes do they have web bugs on their site.
What is a Trojan horse?
In it the Greeks give a giant wooden horse to their enemies, the Trojans, supposedly as a peace offering. But after the horse is inside the city walls, Greek soldiers sneak out of the horse's hollow belly and open the city gates, allowing their soldiers to enter and capture Troy.
Similarly, what is known as a Trojan horse is a destructive program disguised as a simple application or useful program. The program contains additional hidden code which allows the unauthorized collection, exploitation, falsification, or destruction of data. Though lumped in with viruses, it doesn't propagate itself like a virus does.
One of the worst kinds of Trojans is a program that claims to rid your computer of viruses but instead puts them on your computer. Another common type will open a security hole for a hacker to get into your system and do all kinds of nasty stuff
You can prevent getting a Trojan Horse by having up-to-date anti-virus software, not opening unsolicited attachments, and scanning freeware and shareware you download before you unzip.
What kind of attachments can contain viruses
A:
Basically, anything that is executable (able to run). You're generally safe with picture files, text files, and the like. However, it still pays to scan first.
Here is a very short list of the most common to watch out for:
exe
zip
scr
vbs
bat
com
pif
asp
doc
xls
Those are the most common; don't misinterpret this to mean that if you get one of these files it's automatically a virus. A ".zip" file may be nothing more than a set of compressed files your friend sent you to look at. A "doc" or "xls" file may simply be an MS Office file. It's just that these type of files could also be viruses.
A couple other virus tricks you need to be aware of. First, MS Office files can contain what are called "Macro Viruses". Without getting into too much detail, these files can run a "macro" (a macro is a mini-program run from within another program) that can be as destructive as any "regular" virus. So, my advice on MS Office attachments is not to open them unless you are expecting them. If they just show up, verify with the sender first.
The other trick you need to look out for is an extra extension added to an attached file. For example, you may have something like "mypicture.jpg.vbs". In fact, if you don't have your computer set to view file extensions, it may just look like "mypicture.jpg" and omit that last "vbs" part.
This may appear to be a jpg picture file, but it's actually a Visual Basic Script file. If executed, it will happily infect your computer with a virus.
So, be careful out there.
Scan Individual Files for Viruses
We often recommend that when you download files you should scan for viruses. The same is true for attachments sent via email (although, if you have decent antivirus software this should be taken care of).
But do you need to scan your whole computer? No, you can scan just one file with most AV software (Norton, McAfee, AVG).
Just right click the setup file and you should see "Scan with whatever AntiVirus" or just "Scan for Viruses".
Your AV software will come up, scan the file, and let you know of any infections.
Some newer AV also lets you scan zipped folders before unzipping.
Knowing that it's safe, you can then proceed with installation of the program or opening the file.
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