Form-level validation is when you perform data validation after all fields on a
form have been completely filled in by a user. For example, a customer entry
form could require a user to fill in a name, an address, a phone number, a city, a
state, and a zip code. After the user fills in these fields and clicks an OK button,
validation takes place to verify the data in each field.
Field-level validation is when each field is validated as it is filled in—in other
words, fields are validated one at a time. For example, a user could type in a
value for a zip code field on a customer entry form, and validation of the zip
code field would occur either as the user types in the value or before the user
moves to another field on the form. In Visual Basic, you can use the Change
event for a TextBox control to verify that the zip code value is numeric.