Basically you have Testers and Developers.
Testers can do everything in the project, including serving as administrators. Developers can only edit/create issues, and they can see (read only) all the rest of the system.
Within Testers, your account either be set up as an Enterprise Account or a Professional Account (and this will affect all its testers). The difference between Enterprise and Professional, is that the Enterprise can also use all the integrations (bug tracking, automation, API, etc). Both types of testers have full access to all the features within the system. You can also read more about this form our pricing page.
I hope this information is useful to you.