Potholes are, literally, holes in the road.
Manholes, however, cover holes in the road.

Potholes are also called kettles or chuckholes.
Manholes, however, go by the mundane names of inspection chambers or utility holes.

I kind of hate potholes. They spoil an otherwise smooth and uninterrupted drive.
Manholes, however, are OK when level with the road; else expect the vehicle and occupants to jive.

Potholes become bigger with time (you see and feel their presence) as vehicles knock off pieces of tar.
Manholes, however (again, when level with the road), are unobtrusive beneath your car.

Potholes (the deep and bad ones) make you feel as though you are on a bucking bronco—you go up and down, and weave from side to side.
Manholes, however, are best not left open—especially during high tide!!!

Potholes are an on-the-surface system of troughs that cause chaos and discomfort.
Manholes, however, are an underground system of pipes that represent order (and dirt!).

[The first picture is that of a pothole. The second is that of a manhole.]