What if no Role is assigned to a User in salesforce?

Yes,if no role is assigned , the user will get access through
1.Object's OWD specified( private, read only etc..).
2.Sharing Rules: Remember you can share the records with Public Groups; and you can add users in Public Groups.      (setup->manage user->public group)
3. Manual sharing on the records.
While creating a user we can create user without Roles, but actual functionality was explained below:
Roles (configured under Admin Setup->Manage Users->Roles) are used to control which Salesforce records are available to users assigned to the Role. Unlike Profiles, there is no configuration in a Role.Instead, Roles are arranged in a hierarchy.Top level Roles can contain lower level Roles.Record access is determined by the Role of the Record Owner. Any member of the Role hierarchy above the Record Owner’s Role will have access to the record. Users in the same role as the Record Owner, and users in roles outside of that hierarchy, may or may not have access.By default, all Users have Read-Write access to all records in Salesforce.
However, this is controlled by Organization Wide Sharing — a topic for another time.
I hope it will help you.