The User-Agent string format for human-operated web browsers

The User-Agent string format is currently specified by Section 14.43 of RFC
2616 (HTTP/1.1) The format of the User-Agent string in HTTP is a list of product tokens (keywords) with optional comments. For example if your product were called WikiBrowser, your user agent string might be WikiBrowser/1.0 Gecko/1.0. The "most
important" product component is listed first. The parts of this string are as follows:

Product name and version (WikiBrowser/1.0)
Layout engine and version(Gecko/1.0). In this case, this indicates the Layout engine and version.

During the first browser war, many web servers were configured to only send web pages that required advanced features
to clients that were identified as some version of Mozilla.[citation
needed] Other browsers were considered to be older products such as Mosaic, Cello or Samba and
would be sent a bare bones HTML document.

For this reason, most Web browsers use a User-Agent value as follows: Mozilla/[version] ([system and browser information]) [platform] ([platform details]) [extensions]. For example, Safari on the iPad has used the following:

Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Mobile/7B405

The components of this string are as follows:

Mozilla/5.0: Previously used to indicate compatibility with the Mozilla rendering engine
(iPad; U; CPU OS 3_2_1 like Mac OS X; en-us): Details of the system in which the browser is running
AppleWebKit/531.21.10: The platform the browser uses
(KHTML, like Gecko): Browser platform details
Mobile/7B405: This is used by the browser to indicate specific enhancements that are available directly in the browser or through third parties. An example of this is Microsoft Live Meeting which registers an extension
so that the Live Meeting service knows if the software is already installed, which means it can provide a streamlined experience to joining meetings.

http://en.wikipedia.org/wiki/User_agents