Ignore won't include the column in the UPDATE statement at all if left blank. This is good for encrypted passwords, since you can't fill the form element with the existing password on an update page and would want the password to stay the same if a new one wasn't entered.
Blank will use the literal empty string.
Default will set the value to the default value for that column as defined in the database itself
Null will use null instead of an empty string in the database (the column would have to support using null values)
Zero is good for checkboxes bound to tinyint columns
Usually I'll use blank or null for most, but ignore for password fields.