What is xUnit and NUnit?
To put in simple words, xUnit is an extremely popular open-source and community-focussed unit testing tool for testing .NET applications. It was created by the original inventor of NUnit v2.
You can download the xUnit package from nuget.org. The latest version of xUnit is 2.4.1. It also supports parameterized testing with Selenium. [Fact], [ClassData], [MemberData], [Theory] are some of the widely used annotations in xUnit.
nUnit is a popular open-source testing framework ported from JUnit. nUnit has received more downloads than xUnit. The latest version of NUnit is NUnit 3. NUnit can be used for performing test driven development in C#.
Check xUnit vs NUnit for more information.