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For practical examples of how hooks can be employed, see the following modules:

1. Submission Pre-Parser module. At its core, it's fundamentally just a visual wrapper for writing your own code hooks. It registers a single code hook for each of the ft_process_form, ft_api_process_form and ft_update_submission functions. When those functions are executed, the module processes each and every "rule" (i.e. PHP code) that has been added through the UI.
2. Extended Client Fields module. This module uses both code and template hooks to insert new fields into the edit client pages - both administrator and client - which are saved to the database by attaching custom events to the functions that get called when the form is updated. If you want a good example of how to combine the two hook types, this is a good place to look!