Both دكتور and طبيب seem to be interchangeable. As in many other cultures, medical practitioners are usually called 'doctors' in the Arab world, even though they don't necessarily have a doctorate. Please feel free to correct me.
In standard arabic Tabib is used for 1. all people who cure other people ( doctor, practitioner, physician,nurse..etc)
Dktoor applies only for people with PhD degree.
In spoken arabic Tabib is used RARELY, we use Dktoor instead of Tabib for both cases