The FormAPI, first introduced in Drupal 4, is a very powerful API that allows you to create forms effortlessly. Every major release of Drupal, the FormAPI has become more mature and flexible, allowing you to create full forms with fields, and provide ways of altering them on output with #after_build, or hook_form_alter, for example.
In Drupal 7, the FormAPI has a 'date' type form element. This field generally is used to collect data such as a birthdate, or a date in the future. By default, it provides a month, day, and year option as 3 inline select drop downs.