In W C Iowa, a disc has a set of round curved DISC blades that turn on a shaft mounted thru bearings. A harrow usually has teeth mounted on a bar. The teeth can be of round spring steel bolted to bar, a steel bar bolted or welded to bar, or of flat spring steel bolted to bar. 

BUT have seen what I call a disc listed as a harrow on online auctions. 

I've Some older literature that calls my "disc" a "disc harrow". 

The beginning of modern farming if used to break the soil the first time for the growing season seem to be called a plow. Be it moldboard or chisel plow, or disc plow, and  other variations yet I am sure. The next step seems to be harrowing, being it a spike tooth, spring tooth, or disc all called a harrow it seems a 100 years ago. So anything generally used for the secondary tilling became a harrow of some type.

I don't know other places but in the USA we seem to always shorten the official name of things. But even today the USA is regionalize and the common name used for farm tools  changes from region to region. With the world wide web things are getting more universal. But add English speaking countries it gets real confusing, like the Aussies calling a combine a header. And there grain seeder, what I call a drill has some name that always makes me stop and think (no getting a foggy idea of there term now).

And tools only used regional , like a go devil is mystery to many me included. I know it was used on weeds in row crops so is it a regional name of row crop cultivator or still different?

But from the post asking about a harrow on this board you have to wait and see if you can figure out if they mean what has generally become known as a disc or the spike tooth harrow I think of first.