Error values, such as #N/A, #VALUE!, #REF!, #DIV/0!, #NUM!, #NAME? or #NULL! appear in cells when a formula is incorrect and cannot be calculated. Error values always begin with a number sign (#)
#DIV/0 - Division by zero
The division operation in your formula refers to a cell that contains the value 0 or is blank.
#N/A - No value available
Technically, this is not an error value but a special value that you can manually enter into a cell to indicate that you don't yet have a necessary value.
#NAME? - Excel doesn't recognize a name
This error value appears when you incorrectly type the range name, refer to a deleted range name, or forget to put quotation marks around a text string in a formula.
#NULL! - You specified an intersection of two cell ranges whose cells don't actually intersect
Because a space indicates an intersection, this error will occur if you insert a space instead of a comma (the union operator) between ranges used in function arguments.
#NUM! - Problem with a number in the formula
This error can be caused by an invalid argument in an Excel function or a formula that produces a number too large or too small to be represented in the worksheet.
#REF! - Invalid cell reference
This error occurs when you delete a cell referred to in the formula or if you paste cells over the ones referred to in the formula.
#VALUE! - Wrong type of argument in a function or wrong type of operator
This error is most often the result of specifying a mathematical operation with one or more cells that contain text.
When a formula yields an error value (other than #N/A) in a cell, Excel displays a green triangular error indicator in the upper-left corner of the cell and an alert options button appears to the left of that cell when you make it active.