A Ranger is a user account that identifies a person and is tied to a login.
A Park is an organization acount, which will be tied to a credit card if it opts for a paid tier. Parks can have multiple Rangers tied to them, and a Ranger can be part of multiple Parks.
Trails are a way to group your API Traffic - we recommend one Trail per environment at the least.
Finally, Tracks are the API Traffic that gets captured to a Trail and what make Debugging so much easier!