Microsoft's version numbering identifies the specific release and revision of Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 software products. If you are running Microsoft's Internet Explorer

Internet Explorer

Windows Internet Explorer is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year...

 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

Software build

In the field of computer software, the term software build refers either to the process of converting source code files into standalone software artifact that can be run on a computer, or the result of doing so...

 number, and the fourth number is a complex number that includes the Service pack

Service pack

A service pack is a collection of updates, fixes or enhancements to a software program delivered in the form of a single installable package. Many companies, such as Microsoft or Autodesk, typically release a service pack when the number of individual patches to a given program reaches a certain ...

 release, International Client pack, and hotfix

Hotfix

A hotfix was originally the term applied to software patches that were applied to live i.e. still running systems. Similar use of the terms can be seen in hot swappable disk drives...

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 in 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

This convention has changed slightly with Windows Vista SP1. With Windows Vista the build string is shown as 6.0.6001.18063 with the last digit (x) of the 600x number representing the service pack (ex: 6001 = SP1, 6002 = SP2) and everything after as the exact build number.

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.