When creating a custom form, how does one store the data in drupal db and then retrieve for display?
A specific answer: you can use sumbit hook to store data. see Submitting Forms: http://drupal.org/node/751826

A general answer: follow this guide related with creating, theming, validating, submitting, storing data, and understanding the flow in Drupal 6 forms: Form API Quickstart Guide http://drupal.org/node/751826

to retrieve data you can use these guidances:

db_query documentation http://api.drupal.org/api/drupal/includes--database.pgsql.inc/function/db_query/6

Database abstraction layer http://api.drupal.org/api/drupal/includes--database.inc/group/database/6

also you can review Examples module http://drupal.org/project/examples specifically the module dbtng_example to database examples or page_example to build a static page add your retrieved information. please note that dbtng_example module is only for Drupal 7, but reviewing this module you will have more ideas about what to review in Drupal 6.

also could be useful the "Module developer's guide" to dive in tutorials/examples about how build forms and retrieve data using modules http://drupal.org/developing/modules