I am adding a content type in Drupal 7. I want to add a simple node-reference field. Isn't this included in Drupal 7?
I agree with H.NiK both modules can help you to accomplish what you are looking for. However, I tend to think that the Relation Module is used for complex relationship. I use Entity Reference because it is exactly what I need because I don't need to use very complex relationship. You can find in this question some opinions about those modules. I will not recommend Reference because it will be deprecated in favor of Entity Reference. They mentioned in the page module.
You can find how to use entity reference from several sources one of my favorites is nodeone. Here are the links:
Use the References module:
This project provides D7 versions of the 'Node reference' and 'User reference' field types, that were part of the CCK package in D6
or Entity reference:
Provides a field type that can reference arbitrary entities
Relation is an API module and storage model for both simple and the most complex relations between entities. The module can handle both directional and symmetrical relations very well.