Microsoft: Microsoft Version Number

A Microsoft Version Number is the number that identifies the specific release and revision of Microsoft software products. If you are running Microsoft's Internet Explorer you can view this number by selecting "Help" and "About" from the menu. This version number is composed of 4 parts separated by periods and may include alpha characters. The first number is the primary version number, the second number is the secondary version number, the third number is the build number, and the fourth number is a complex number that includes the Service pack release, International Client pack and hotfix numbers.

Primary and Secondary Version Number
As Microsoft develops a software product new functions or features are added, major new features may be indicated with a new primary version number, minor functions or features will increment the secondary version number.
For example: Suppose I am currently running Internet Explorer version 5.50.4807.2300. The primary and secondary version is 5.50. The build number is 4807 and 2300 identifies service pack 2 and hotfix 300.

Build Number
Higher version and build numbers are always later and will not be overwritten with earlier version and build numbers during a Service Pack installation or upgrade.

You can also identify service pack, international client pack, and hotfix numbers from the fourth element of the version number, as described below.

Service Pack
The leftmost digit, .x000, is the service pack release number (for example, 2.00.1239.2000 denotes Service Pack 2). By default service pack 2 will contain all of service pack 1. Microsoft documents all components of each service pack on their website http://support.microsoft.com

International Client Packs
The ICP version number is represented by the second digit, which can range from 4 to 9 (.0400, .0500, .0600, .0700, .0800, .0900). Microsoft supports the following languages with International Client Packs: Arabic, Chinese (Simplified), Chinese (Traditional), Czech, Danish, Dutch, English, Finnish, French, German, Greek, Hebrew, Hungarian, Italian, Japanese, Korean, Norwegian, Polish, Portuguese (Brazilian), Portuguese, Russian, Spanish, Swedish, and Turkish.

Hotfixes
The last three digits are the hotfix version number (which can range from .x001 to .x399). Hotfixes are accumulated and may be included in the next service pack.

note: A service pack version will overwrite the ICP version until the new service pack ICP build is released.

N.B. Your version number may vary depending on IE version, service pack, hotfix and release number.