Here's a good comparison of the two: interface vs abstract class. I've copied a specific example from there below:
Interface
Interfaces are often used to describe the peripheral abilities of a class, not its central identity, e.g. An Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.
Abstract class
An abstract class defines the core identity of its descendants. If you defined a Dog abstract class then Dalmatian descendants are Dogs, they are not merely dogable. Implemented interfaces enumerate the general things a class can do, not the things a class is.
