Difference in Boto3 between resource, client, and session?

I'm learning how to use AWS SDK for Python (Boto3) from the following resource:
https://boto3.readthedocs.io/en/latest/guide/quickstart.html#using-boto-3.

My doubt is when to use resource, client, or session, and their respective functionality.

I am using Python 2.7.12 in Ubuntu 16.04 LTS.