This document lists all the situations where WordPress sends an email, along with how to filter or disable each email.
This documentation has moved here: https://github.com/johnbillion/wp_mail
The WordPress Customizer is an interface for drafting changes to content while previewing the changes before they are saved. This is an alternative to the "save and suprise" model of changing settings without knowing what exactly will happen.
The customizer can be accessed in the admin interface under Appearance > Customize.
#19909 is the trac ticket that introduced the Customizer during the 3.4 release cycle.