Declare page and data pages both are same. In 6x it is declare page where as in 7x it is data page. And in pega 7 it came up with additional features.
Here is an example, hope this helps you.
Let's say, I want to fetch the customer data from an external source and would like to use the fetched data in process of building my logic or to display the data in a form.
In the above scenario, in 6x, we usually refers the Declare page in the Pages & Classes tab, and then we apply a filter to get the data of the respective customer and then we will copy the data into other Page or what ever it may be based on the logic or the requirement demands.
But in 7x, we can have a Data page which brings the Customer Data from an external source and make it parameterized let's Customer unique ID.
Now we can have a Page property for the Data class. So that we can directly make a Reference to the Data Page by passing a property(Customer unique ID) as a value. So, in run time the created page property contains whole Customer info.