Most of Drupal's installation is done using a task system. In this system, Drupal proceeds through a series of steps (tasks). Each task is responsible for directing the installer to the next task, like a chain.
Note
This is an important point: It is the responsibility of a task to redirect the installer to the next task. So a task can logically determine what the next task should be, based on its current state.
To get a detailed idea of how the task system works the best code to look at is install.php, located in the root Drupal directory. The install_tasks() function is particularly helpful for understanding tasks.
For example, Drupal performs the profile-install task, which is responsible for preparing for the installation of modules listed in philosopherbios_profile_modules(). Once this task is done, it directs the installer to the next task: profile-install-batch. This task does the actual installation of modules. A few more minor tasks are done before the configure...