A Role is a collection of permissions that determines what a user is able to do. Permissions are always additive, so users start by being able to do nothing, and then each permissions grants them further ability.
The settings for Roles can be found within the main Settings section found always in the top header of all pages.
There are a bunch of included system default roles, or you can create your own roles and choose which permissions they allow.
The list of permissions is pretty exhaustive therefore you should find a set of permissions to suit all Roles within your business.