api_channel_fields settings methods

Regarding the field setting methods found in api_channel_fields

1. set_settings
2. include_handler
3. setup_handler
4. apply

I’ve read through http://ellislab.com/expressionengine/user-guide/development/api/api_channel_fields.html but could use a few clarifications on how I would probably use these methods as a developer.

I’ve been running into these methods as I look over the api’s on the current project I’m working on and could use some expert insight on these methods. Like

are these required? When would you need to know the field_type? What’s the difference between include_handler and setup_handler? Once I have a handler setup, what can I do with it?

any help would be appreciated.

thanks,