In MySQL, a NULL value represents the absence of data in a field, indicating that the field is empty. It is used to denote unknown or inapplicable values. Conversely, a zero value is an actual data value representing the numerical 0. Logically, NULL is not equal to any other value, including zero, even when comparing two NULL values. This means that any operation or comparison involving NULL results in NULL. In contrast, a zero value is treated as the specific numerical value 0 during comparisons and can be directly used in calculations and comparisons.