Great overview of how it all works from Mike Blumenthal.

Moz Academy: How Google Local Works - Moz

ver wonder where this whole Google Local thing came from and how it’s changed over time? Yes? Well then you’re in luck! We have Mike Blumenthal (aka Professor Maps) from Get 5 Stars here to give you the rundown.
Brief History

Understanding how Google Local works is important. There's a lot of misunderstanding about how they assemble and display a record. If you misunderstand this, you end up focusing on the wrong things.