Visual Studio 2015 includes a lot more than just an updated version of Microsoft's integrated development environment. It also encompasses the "Roslyn" .Net compiler platform, ASP.NET v.Next (codenamed Project K) and Apache Cordova tooling support, among other features. There are updated versions of VB.NET, C#, C++, TypeScript, F# and other languages and tools in VS 2015, as well.
Microsoft is positioning Visual Studio 2015 and .NET 4.6 as "a great toolset for any developer targeting any device on any platform," in the words of Soma Somasegar, Corporate Vice President of Microsoft's Developer Division.
VS 2015 is also a key piece of Microsoft's Windows 10 app story, as it is meant to be the crux of the toolset for developers building Universal Windows apps. However, the additional Windows 10 tooling that developers need to build those apps is still not yet available in final form; it's still in preview at this point. The expected RTM date of that tooling is July 29, the day Windows 10 starts rolling out.
In March, Microsoft released the pricing and version line-up for Visual Studio 2015. There are three primary Visual Studio 2015 versions. Microsoft combined Visual Studio Premium and Visual Studio Ultimate into a new single bundle called Visual Studio Enterprise with Microsoft Developer Network (MSDN). The new The Enterprise version sits alongside Visual Studio Professional with MSDN and the free Visual Studio Community SKUs. (The Visual Studio Community SKU is aimed at individual developers and developers in small shops.)