Structured citation is an organized listing of your business with NAP essentials and you have control over the listings. If you have changed your business address or phone number, you can always update your business information making it consistent. When you setup a Google My Business or Yelp business page, you can at any time update your citations.
Unstructured citations mostly mention your business name not exactly the way you wanted it but may talk about your business. You may not have control over the listings as it is cited on a log or news portal by a third party.