Gemfile.lock is an important file for deployment, because it contains specific details about the gems your app needs.
Heroku.com uses a database called Postgres. To connect to a postgres db from our Ruby code, we use a gem called pg. I put it inside a group called 'production' because I don't want to install this stuff on my own computer.
a tag is connected to a picture by a Tagging object. The Tagging stores the picture's id and the tag's id.