Aperture priority – Usually labelled A or Av. Use this mode when you want control over depth of field, or how sharp your photos look at varying distances from the plane of focus.
Shutter priority – Usually labelled S or Tv. Use shutter priority when you want to prioritize capturing something in motion like for wildlife, kids sports, people, or vehicles.
Program mode – Similar to fully automatic mode, except you can adjust the aperture and shutter setting combination for the desired effect and still achieve proper exposure. This is good for when you just want to make sure you get a properly exposed shot, with flexibility when needed.