What?

A static class is very similar to a non-static class, however there's one difference: a static class can’t be instantiated. In different words, you cannot use the new keyword to make a variable of that class type. As a result, there's no instance variable, you access the static class members by using class name.

public static class MathUtility  
{  
    public static int Add(int a, int b)  
    {  
        return a + b;  
    }  
}
//We can call it in the following way:
int result = MathUtility.Add(2,5);

A static class will be used as a convenient instrumentation for a set of ways that simply treat input parameters and don't get or set any internal instance fields. For example, in the .NET Framework class Library, the static System.Math class contains functions / methods that perform mathematical operations, with none of them demand to store or retrieve knowledge that's distinctive to a specific instance of the Math class.

Features of Static Class

1. It can only have static members.
2. It cannot have instance members as static class instance cannot be created.
3. It is a sealed class.
4. As static class is sealed, so no class can inherit from a static class.
5. We cannot create instance of static class that's the reason we cannot have instance members in static class, as static means shared so one copy of the class is shared to all.
6. Static class also cannot inherit from other classes.