What is a User Agent string, also known as your Browser string? Well it's basically your computer's information as far as what software you are running. It just gives the basics as to which OS (operating system) you are running, your basic security (if you have any) and which internet browser you are using. It also may tell if some other applications are installed, like the Microsoft.NET framework and the version you have installed.

Sometimes 3rd party programs will also inject some sort of tag in it to show that you are running their software as well. The Google Toolbar adds a small "GTB6" tag for example.

Why does your browser give away this information? Some websites may make use of it to help them show you certain pages optimized for your particular setup, is one example. In general, it is used by websites simply to enhance the user experience in one way or another. It can be used to help identify you, but generally not without some sort of other useful information, such as your IP address, since thousands, or hundreds of thousands of users may have the same exact User Agent string as you do.

How do you find your User Agent String? Go to IPCHICKEN.COM

Here is a great site that will help you read user agent strings: CLICKY

Here are some examples:

My main computer at home:

On ipchicken it will be the last line where it starts off as Browser. ipchicken reads mine as:
Browser: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7

User Agent String explained:

Firefox 3.5.7
Mozilla	It's a Mozilla based browser
5.0	Mozilla Version
Windows	Platform
U	Security values: N for no security U for strong security I for weak security
Windows NT 6.1	Operating System: Windows 7
en-US	Language Tag, indicates the language for which the client had been localized (e.g. menus and buttons in the user interface) en-US = English - United States
rv:1.9.1.7	CVS Branch Tag The version of Gecko being used in the browser
Gecko	Gecko engine inside
20091221	Build Date: the date the browser was built
Firefox	Name : Firefox
3.5.7	Version

Here is what it shows from my iPod Touch (it would look almost the same from a iphone)

When I go to ipchicken it shows: Browser: Mozilla/5.0 (iPod; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7D11 Safari/528.16

User Agent String explained :

Safari 4.0
Mozilla	It's a Mozilla based browser
5.0	Mozilla Version
iPod	Apples iPod
U	Security values: N for no security U for strong security I for weak security
CPU iPhone OS 3_1_2 like Mac OS X	Operating System: iPhone OS Version 3_1_2 Based on Mac OS X
en-us	Language Tag, indicates the language for which the client had been localized (e.g. menus and buttons in the user interface) en-us = English - United States
AppleWebKit	The Web Kit provides a set of core classes to display web content in windows
528.18	Web Kit build
KHTML	KHTML
like Gecko	like Gecko
Version	Safari Version
4.0	Version
Mobile	Mobile Device
7D11	iPhone Firmware Version
Safari	Name : Safari
528.16	Browser build

Here is my archos media table:
Browser: Opera/9.02 (Linux armv 7l; U; ARCHOS; GOGl; mobile; G6H; Version 1.0.87 (WMDRMPD: 10.1); en)

User Agent String explained :

Opera 9.02
Opera	Name : Opera
9.02	Browser version -> old version, for compatiblity reasons only. See Version below
Linux armv 7l	Operating System: Linux
U	Security values: N for no security U for strong security I for weak security
ARCHOS	???
GOGl	???
mobile	???
G6H	???
Version 1.0.87	???
WMDRMPD: 10.1	???
en	Language Tag, indicates the language for which the client had been localized (e.g. menus and buttons in the user interface) en = English

Trackers and websites use useragent strings to help identify the person. You can go to a proxy site but in the end your user string will stay the same. There is an extension for Firefox to change your UserAgent string on the fly named UserAgent Switcher.

There are many lists on the internet where you can find some UserAgent Strings to add to the list of UserAgents to switch between. For example, here.

Some fun tricks can be done when playing with UserAgent Strings. Want to see how a website might look strictly to a GoogleBot? Some websites optimize their pages to show only a certain page if it detects a GoogleBot, just copy the GoogleBot user agent string to your UserAgent Switcher and select it, then load the page.

There have also been some tricks to bypass password protected sites or pages by masquerading as a search engine crawler by using a search engine's UserAgent String, although in general, this isn't usually likely to happen.

Know of any other cool tricks by fiddling with UserAgent Strings? Let us know in a comment!