Beginning Table Views · Creating a Model | Ray Wenderlich

A way to think about delegates and data sources is that data source methods ask information about the data itself. For example:

How many items are being displayed? How many sections are there?

Delegate methods respond to actions. For example:

The user tapped a row
The user swiped to delete

What you do with your data is up to you although it is a good practice to be consistent where you modify your data.