Difference between *device descriptor* and *device handler*
Where did you find that terminology? Check how it is defined there.

If it was up to me, a "device descriptor" would be a way to describe a device, perhaps a data structure giving details on it (or some sort of reference to it, like a pointer or an index into an array of the above).

As you state, in my book a "device handler" would be some sort of routine (or a set of routines) to manage it, a driver.