Admins manage your Skedulo apps and can configure them using admin settings. Usually, they are also your Salesforce system administrator and maintain your users.
Schedulers are there to book appointments and make sure that resources are assigned (or allocated) jobs so that your customers get the service they need from your business. These users mostly use Salesforce and the Skedulo web app to do their job.
Resources are out in the field, visiting customers and usually only need to use the Skedulo mobile app.
Skedulo users are defined during step 1 of onboarding.