Exploring the root of the word 'arboretum'

It’s on one of those signs you see along the highway, or in a listing on a map or travel guide: "National Arboretum,"  "University Arboretum," or perhaps "State Arboretum." 

This invites the question: what exactly is an "arboretum?" 

Think of it as a “zoo for trees.” Yes, you read that correctly.