All the hooks are executed after the core WordPress application completes its loading process.
So initialization hooks are mainly used to initialize the process in plugins and themes.
Available init hooks in WordPress, in the order of their execution are -
1)init -> runs after WordPress has finished loading but before any headers are sent. Generally, this used by plugins to initialize their process.
2)widgets_init -> is used to register sidebar widgets of the application. The
register_widgetfunction is executed within this hook.
3)admin_init -> is executed as the first action, when user access the admin section of WordPress. Generally, it’s used to initialize settings specific to the admin area.
admin_init hook runs every time admin page is rendered.
You can also look into Wordpress Codex for more information about activation process.