Difference between string.Equals() and "string".Equals()

I am new to C# and found two ways to compare strings. Whats the difference between these approaches?

var a = "hello";
var b = "hello";

a.Equals(b);
string.Equals(a, b);

>Solution :

The difference is that a.Equals(b) throws an exception if a is null:

string a = null;
string b = "hello";

a.Equals(b); // throws a NullReferenceException
string.Equals(a, b); // returns false

Especially when you know that the strings could be null (for example user inputs) it’s safer to use string.Equals(a, b).