well, that's the whole point of web bugs. your information is automatically sent to a third party if that web bug is owned by a marketing outfit like DoubleClick. the gif is not hosted on the machine hosting the web site.
I'll show you what I mean. Go to this page: www.interpactinc.com/bugs.html
the three images on that page are housed on another machine, not the machine hosting interpactinc.com. I will have a record of your machine accessing Interpactinc.com. I will also have one of your mchine access reeza.com and inforwarcon.com. I could take images from www.leftbusinessobserver.com and encode their addresses into that page and you would end of visiting the machine that hosts doug's pages at panix.com.
if you look at the bar at the bottom of your browswer, you'll see the addresses for the images.
web bugs are so small that they take no time to load and so you have less of a chance to see that they're not housed to the machine you think you're accessing. This is a "normal" technology. Akamai uses it for images that you see on CNN.com for instance.
the info they send is the info that machines communicate with.
I want to access a site, my machine knocks on the door of that machine and says, "Hi, my IP address is xx.xxx.xx.xxx. I'm using Netscape 4.75, it's frames capable, and my monitor is 17". I also have RealRadio, windows media player, and I am Flash enabled."
I can look in the logs to the web page i put up and posted here yesterday and find out who visited that page--if I know their ISP and where they live--and they used their ISP without an anonymizer
all of us send this info each and every time. It is how the 'net works. Now, the issue comes in when you access a bug. Go into your Windows Explorer. Go to Windows folder on the C:/ drive. Open the Temporary Internet Files folder. There, you'll find every image you ever browsed on a page. Even if you eyes didn't see that image, it's in there. What's also in there are 1x1 pixel gifs, invisible to the human eye. I have some 5x5 gifs on the pages I create-- they're used to make graphics/text placement consistent no matter what browser is viewing the page.
Those same small invisible gifs are used for Web bugs. They look like this in the HTML:
<img src="http://ad.doubleclick.net/ad/pixel.quicken/NEW" width=1 height=1 border=0> <IMG WIDTH=1 HEIGHT=1border=0SRC="http://media.preferences.com/ping? ML_SD= IntuitTE_Intuit_1x1_RunOfSite_Any&db_afcr=4B31-C2FB-10E2C&event= reghome&group=register&time= 1999.10.27.20.5 6.37">
the image isn't on the page you're viewing, it's on doubleclick.net. so, if you visit a page with one of these, you're machine is actually accessing two machines.
So, this place, Doubleclick, if it has web bugs at loads of sites (and they do) can --with the help of cookies--track you as you move from EPI.net, to HotBabes.com to Psychoticsanonymous.com to the Singlematch.net where you start surfing the Kinky sex ads.
This is fabulous info for people who want to know HOW people use the 'net and what kinds of things they're interested in.
The real fun comes when you start registering for things in order to access content. As Marco said, this is how they are actually paying for those sites. If I register at BigAssBank.com for information about home loans I will likely use my real name, my real address, my real phone number, real whatever. Now, BigAssBank has information about you associated with your IP address and all the other info mentioned above.
If your IP address is 207.15.182.142, then BigAssBank has the information to create a file on your IP that associates you IP with all the information you provided to them while logged in thru your machine.
If BigAssBank.com were to sell this information to BigAssMarketingHaus.com it can now access databases of information with your real name, real address, and all the info that marketers keep on you: demographics and consumption habits.
Now, they can connect your demographic/consumption profile in "real" life to your habits on the 'net--whenever you got to a page with BigAssMarketingHaus.com's web bugs and cookies.
you'll notice that you're getting spam mail after you visit some highly commercialized site. If that site is using these technologies and if they've bought a database with IP address and matching email addresses, then you'll get spam when you check your mail right after visiting a site.
That's how they're doing that: web bugs and cookies.