Class vs. Interface

Question

I have a quite basic question:

When should we decide to use Interface or Class for a specific class?

For example: says we have 2 classes, Customer and Doctor

Answer 1

Parent Class is the one which will have bare minimum properties common to all of its sub classes.

But Interface is a contract which tells its implantations to provide if it is not an abstract class.

And the One important difference between a class and interface is that

class inheritance will give relation between two common subclasses.

Where as Interface implementation gives a relation between two uncommon classes.