Structure

This refers to the way that the different items of interest to the business (data entities) are inter-related. An example will help get across what I mean. Watch for the entities Customer, Order, Order-line, Product.

So here I am, a small business, PhotoReady, selling stock photos and other products to web customers. A customer can buy many items in one order (i.e. s/he can add lots of things to their shopping basket). There is one delivery email address per order but potentially (hopefully!) many items per order. And of course each line on the order could be for more than one of a particular product. Good customers come back and place more orders, and a particular product can be sold to many customers.

The types of data and their inter-relationships are static, that is as long as this aspect of my business doesn't change, neither will the structures or types of data.

By the way, you probably spotted that I didn't say that email address was an entity - that's because it's an attribute of Customer. Other examples of attributes would be Product-Description on the Product entity, Quantity-Ordered on the Order-Line entity.