What is the difference between a "good" and a "bad" oxide?
It depends on the application and the material and whether you mean the native oxide or deliberately depositied oxides. A 'bad' oxide would be something harmful to the efficiency or lifetime etc. of the device. A good oxide would provide some beneficial effect, such as protecting the device from atmospheric contamination or acting as a dielectric,
This is rather vague without proper context. You also shouldn't be lazy in writing a bit more than just this terse sentence.
Take a look at an "aluminum oxide". Do you think it is bad? What if I say that Al2O3 (also called alumina, which is technically an aluminum oxide) is one of the most useful substance around. Or what about a metal oxide layer that is used as an insulating barrier, especially in tunnel junctions?
But an oxide can also corrode, reduced conduction and electrical contacts, etc...
In other words, pay attention to what you want to do and what things are being used for.
Separate names with a comma.