There is no difference between a cap and a crown. Dentist will normally use the term crown. While people will sometimes refer to a crown as a cap. They are one in the same. The difference would be what type of crown you want to have. These would be the two most common.
All-Porcelain Crowns General Description
Inlays, onlays, crowns and aesthetic veneers May fracture under heavy biting loads
Strength depends on adequate porcelain thickness; Color and translucency mimic natural tooth appearance Cost is about 10% higher; requires at least two office visits and laboratory services
Porcelain Fused to Metal Crowns
Porcelain is fused to an underlying metal structure to provide strength to a crown or bridge
Crowns and fixed bridges Very strong and durable Including both porcelain and metal creates a stronger restoration than porcelain alone Porcelain can mimic natural tooth appearance, but metal limits translucency. Requires at least two office visits and laboratory services