Difference between ISBLANK and ISNULL

• ISNULL is used for the Numeric & Text field. 
• ISBLANK is used for both the Text as well as the Number field.

ISBLANK	ISNULL
Determines if an expression has a value and returns TRUE if it does not. If it contains a value, this function returns FALSE.	Determines if an expression is null (blank) and returns TRUE if it is. If it contains a value, this function returns FALSE.
Text fields cannot be null. So use mainly for text fields.	Numeric fields cannot be blank. So use mainly for numeric fields.