Ich attacks the eye and can either by in form of a white spot or a cloudy eye. A lot of time people see a cloudy eye on a fish and don't know that it is ich. Ich can and does attack the eye, some time it can damage the eye or even cause the fish to lose the eye altogether. Because of that you should start to treat once you see a fish ich on the eye as it can be very bad.
Pop-eye is not a diseases but a symptoms of different things. Some which can be very minor, like a fish bang it's eye against a rock causing a "black eye" which will make the fish have a pop-eye which usually will go away in a short period. Other could be from a bacteria infection to having trapped gas.