How to save extra data when I save an entry I am trying to understand if it is possible to do a hook on an entry save without creating a plugin.

This is what I would like to happen:
- In my entry, I have some address fields (street, city, province...).
- I would like to take those info and go grab a lat and long from Google maps and save it with the entry.  The lat and long fields are also available in the entry if people want to fill them or modify them.

I was doing all this in Wordpress with custom post types, but I'm trying to convert it to Craft.

Is it possible or do I have to create a plugin to do it?
I think the short answer is yes, you do need a plugin to listen for events or tie into Craft's hooks.

Luckily if that is all you need to do you can probably get away with writing a plugin that is only the singular plugin class file, you may not need any other plugin classes (models/records/etc).

It seems as if you want to listen for onSaveEntry.