The Entity reference module provides a field type to reference entities.
Drupal 6 offered 'node_reference' and 'user_reference' field types. They were replaced in Drupal 7 by the References module. Entity reference is the new approach and handles references to every type of entity.
Because the references are at the entity level, they can appear in all sorts of places and modules can reuse the code everywhere.
This page is a collection of resources about the contributed Entity reference module.