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 4.5.1 is a highly compatible, in-place update to the Microsoft .NET Framework 4 and the Microsoft .NET Framework 4.5.
The Microsoft .NET Framework 4.5.1 runs side-by-side with the .NET Framework 3.5 SP1 and earlier versions, but performs an in-place update for the .NET Framework 4 and the .NET Framework 4.5. Applications that are based on earlier versions of the Framework will continue to run on the version targeted by default.