MSTest is a popular open-source test framework that is shipped along with the Visual Studio IDE. It is also referred to as Visual Studio Unit Testing Framework; however, MSTest is more synonymous within the developer community.
As the MSTest framework comes pre-bundled with Visual Studio, many developers prefer MSTest over other C# frameworks such as NUnit, xUnit.net, etc. for Selenium test automation. The latest version of MSTest is MSTest V2, it is a major overhaul over its predecessor. The earlier version of MSTest i.e. MSTest V1 was not open-source and lacked many good features (particularly parallel test execution) which were supported by other Selenium C# testing frameworks.