Microsoft .NET Framework Redistributable includes all you need to execute applications based on .NET Framework. If one application shows you an error like "Unable to locate required .DLL, MSCORE.DLL", you can fix it installing this component.

The Microsoft .NET Framework 3.0 is the new managed code programming model for Windows. It combines the power of the .NET Framework version 2.0 with new technologies for building applications that have visually compelling user experiences, seamless communication across technology boundaries, and the ability to support a wide range of business processes. These new technologies are Windows Presentation Foundation, Windows Communication Foundation, Windows Workflow Foundation, and Windows CardSpace.