The is a comprehensive software package for developers targeting the .NET Framework 4.5. It includes everything needed to build, test, and run applications that leverage the features of .NET 4.5, including the full reference assemblies, language compilers (C# and VB.NET), and the corresponding runtime components.
It is a common point of confusion.
Released in August 2012, this version introduced several landmark improvements that simplified complex coding tasks: .net framework 4.5 developer pack