The frameworks section of the project.json is confusing me and causing problems with xunit

First: What are, or what are the differences between net451, dotnet54, dnx451,  dnxcore50, I think I also saw dotnet5.4. Compared to previous versions of .NET this part is making no sense to me.

Second: I am trying to add xunit testing for my project. I am following the documentation and the xunit docs. When I created a class library project it had defaulted to net451 and dotnet54 and I had to remove those to get the project to recognize xunit.I then added the dependencies property. I also do not understand why it is added to the dependencies property, but the net451 had packages listed under net451.dependencies. I guess I do not understand what the difference between the dependencies property and the frameworks.net451.dependencies is? I also don't understand when and why you would target a given framework.

Third: I copied the test from the docs:

[Theory]
[InlineData(-1)]
[InlineData(0)]
[InlineData(1)]
public void ReturnFalseGivenValuesLessThan2(int value)
{
    var result = _primeService.IsPrime(value);

    Assert.False(result, String.Format("{0} should not be prime", value));
}

Every time I build VS 2015 Crashes.

Help on these would be greatly appreciated.