what is different between ISBLANK() and ISNULL()?

The difference between  ISBLANK and ISNULL is having to do with text fields.

Use ISBLANK instead of ISNULL in new formulas. 

ISBLANK has the same functionality as ISNULL, but also supports text fields. 

Text fields are never null,

NULL is like a CONSTANT expression which denotes the lack of value

so using ISNULL() with a text field always returns false.

In the case of text fields, you would probably have an empty string like this '' which essentially means that there is some value but since it is a text field the empty string corresponds to no value.

So while ISNULL checks to see if NULL is present, 

ISBLANK checks more specifically based on the type of the field.

For example, the formula field IF(ISNULL(new__c) 1, 0) is always zero regardless of the value in the New field. For text fields, use the ISBLANK function instead.