As of Drupal 5, there are six core required modules (Block, Filter, Node, System, User, Watchdog) and around 23 supplied modules that are usually not activated.
The required core modules are - well - required. They cannot even be turned off - so we need not bother learning about them unless we are into custom module development. The other 23 modules are worth looking into, and they usually will supply most of the basic extended functionality you might need.