Visual Studio is an Integrated Development Environment (IDE) from Microsoft. It enables the developer to work project orientated with various types of projects, including Windows Forms, Console Applications, Office Plug-Ins, and Windows Universal Apps.
The IDE supports various programming languages, the most common being Visual C#, Visual Basic, Visual F#, and Visual C++.
There are several editions of Visual Studio: Community (free), Express (free), Professional, Enterprise, and Ultimate (However, not all are available for all versions).
In order to fully benefit from Code Contracts you need to install the extension for Visual Studio. There's also a Code Contracts User Manual.