To thaw something is to take the time to get it to room temp by moving from freezer to fridge to room taking a few days. To defrost something is to thaw it fast, by running a frozen package under warm water ot putting it in the microwave.
When something thaws, it's a slow, natural, and gradual process. To defrost something, you use something to accelerate the process to make it "un-frozen" like a microwave for example.
Not really a difference it's just like bake and roast it's the same thing pretty much.
Its the same thing