The .NET Framework is Microsoft‘s comprehensive and consistent programming model for building applications that have visually stunning user experiences, seamless and secure communication, and the ability to model a range of business processes. The Microsoft .NET Framework 4 redistributable package installs the .NET Framework runtime and associated files that are required to run and develop applications to target the .NET Framework 4.
The .NET Framework 4 works side by side with older Framework versions. Applications that are based on earlier versions of the Framework will continue to run on the version targeted by default. Version 4.5.1 built upon successes of the previous milestone releases v4 (which among other things enabled developers to optimize their apps for parallel computing or distributed systems) and v4.5 (which for the first time enabled developers to start developing Metro-style apps for Windows 8, Windows RT and Windows Phone software environments).