Google it.
References :

This isn’t a very good example, but here is one of the ebay quality paintings of mine ( I never sell good ones on ebay) on ebay with a shadow on a white background, and because it’s on a white page, you can’t see the excess.
http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&rd=1&item=260168422992&ssPageName=STRK:MEWA:IT&ih=016
That’s the simplest way to do it.

Here is another, http://cgi.ebay.com.au/FM-TRANSMITTER-BK-for-APPLE-IPOD-NANO-VIDEO-30GB-80GB_W0QQitemZ120183771044QQihZ002QQcategoryZ73836QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I think it’s more like what you want. You can also take the photo into photoshop and use the bucket tool to colour in the background white. If you do that make sure to save it as a jpg not a psd or it will be too big.
References :
Accredited ebay powerseller, 7 years experience.

If the object is too big for that, you can use a piece of heavy white paper as a backdrop. Set it up so that it gradually curves from the table top to a prop behind the backdrop. This will eliminate the "horizon" as you shoot. Here is one that is not lit all that great and the background is not white, but you can get the idea of the effect a little better because it is NOT white. Look to see where the background transitions from being flat to being horizontal. http://www.flickr.com/photos/samfeinstein/1023266602/

Here is the way to set it up: http://www.flickr.com/photos/samfeinstein/2039003177/

Here are some more examples with further explanation under the image. Bear in mind that you can use white paper and the background will pretty much disappear. I used the brown paper simply because I had it and because it makes it easier to avoid overexposure, which you have to be careful of with a white background.

http://www.flickr.com/photos/samfeinstein/2039748432/
http://www.flickr.com/photos/samfeinstein/2039748284/
http://www.flickr.com/photos/samfeinstein/2038951527/
References :

This gives the appearance that the item is not sitting on anything, and if you need to cut the item from teh background, you will have a uniform color to extract from.
References :

Or do you mean that there is literally no background behind the item, so that you can place the image on a web page, without having to see a square or rectangular background. In other words, the background of the image is the actual color of the page the image is on?

If the first one, you can simply use a white or black bedsheet or you can get some thicker material from a fabric store. There are also companies that make backgrounds for professional photographers.

If you mean the second scenario, the images are usually saved as a GIF file. The GIF format allows for a "clear" or "no color" background. Most image editing software allows for this.
References :

Photofiltre
http://photofiltre.free.fr/frames_en.htm
Gimp ~photo editor,
http://gimp-win.sourceforge.net/stable.html

..
References :

Another alternative is to shoot your merchandise on a light table. This eliminates the shadows and makes it look as if the background is just pure white. Higher end camera stores can probably set you up with one, but again there’s cost and learning involved.

Or you can shoot your object on a plain white board or wall, but you’ll still get some "background" because of the shadows. Pros use rolls of background paper or "sweeps" to keep from showing where the floor (or table) meets the wall. Depending on the size of your product and the quality you’re going for that professional look can take a long time to master.

Even with the best of equipment, you’ll probably need to fix some things in your photos digitally, so you might as well just go with the software from the start. If you really want good quality, take it to a pro.
References :
Paint.net website:
http://www.getpaint.net/

Example of a light table:
http://www.nextag.com/SMITH-VICTOR-SmithVictor-3-500499380/prices-html