What is the difference between a isBLANK and an isNULL?

Ans:-
Both same thing. The problem is that salesforce made isNULL but it could not check if a blank text field is null. So it used for work for most data types but not strings. So they invented IsBLANK that is capable of checking if string is empty or not. Remember however that even a space or colon is considered as not empty. Anyway in future it is always recommended that we use isBlank.