There are three “priority modes” on most cameras, Shutter Priority, Aperture Priority, and Manual.
Shutter Priority
In this mode you set the shutter speed and the camera takes care of the rest (ISO and Aperture). This is particularly helpful for blurring and freezing motion. The picture below was taken with Shutter Priority Mode.
Aperture Priority
Similar to Shutter Priority, this mode allows you to set the aperture of your choice and the camera picks the apporpriate Shutter Speed and ISO. This mode works well when shooting wide and shallow depths of field. The below pictures were taken with Aperture Priority Mode.
Manual Mode
This is the mode where your picture is truly in your hands. You choose your ISO, Shutter Speed and Aperture. You get to compose your image as you choose. The below picture was taken with Manual Mode.