Difference between "" (Empty String) and isNot Nothing?

I am working on a condition where I have to validate whether the argument is empty or not. Lets assume that argument is Email. I have to check whether the inwards argument Email is empty or not. I can do it in several way but I am not sure which one to proceed with.

I am thinking to check from following statement:

1.Email = "" to check if email is empty string or not. 2. Email isNot Nothing

I wanna know the difference of these two functionality. If there are more function or argument related to validating empty string, You can write that too.

Thanks.