"Milk or water?"
Translation:Sữa hay nước?
Same different :-)
Many people use them interchangeable, but here is the difference:
What do you want? Milk or water? In case, "or" is hay. It's an option.
You can use either milk or water. In this case, hoặc is better for "or." It's an alternative.
Hay has other meanings too so be aware of that. Hay can mean great or habitually/to have a habit of.
Phim này hay quá = this film is great
Cô ấy hay chửi thề = she has a habit of swearing
Normally in a question you would use hay.