What's shutter speed?

Shutter speed is the length of time the camera's shutter remains open.  A long shutter speed will let more light onto your camera's sensor, increasing your exposure.  A short shutter speed let's less light onto your camera' sensor, decreasing your exposure.  If your camera is on a tripod and the scene you're photographing is perfectly still, the shutter speed you choose is largely inconsequential--you can set your aperture and ISO the way you want and set the shutter speed to whatever will give you the exposure you need.

There are two main reasons for taking control of your shutter speed;
  1. Camera Movement. If your camera is not on a tripod (or secured in some other way), it is moving.  When hand holding a camera, you need faster shutter speeds so that the camera's motion doesn't cause blur in your photographs.  A good rule of thumb for lenses without image stabilization (IS) is to ensure you have a shutter speed of the inverse of the focal length of your lens.  In other words, if you are shooting with a lens with a focal length of 100mm, your shutter speed should be 1/100sec or faster.  If your focal length is 400mm, you want a shutter speed of 1/400sec.  It's much easier to hand hold your camera and get sharp pictures with a wide angle lens.  If you're shooting with a 15mm lens, you only really need about 1/15sec to get a sharp picture.  Now with IS lenses, you have much more room to play with.  They usually give you two stops of shutter speed where you can still get sharp pictures--that is, you can set your shutter speed four times slower and still get sharp pictures.  With an IS lens, if you focal length is 100mm, you want a shutter speed of 1/25sec, and at 400mm, you need 1/100sec, and at 15mm, you need 1/4 sec.
  2. Subject Movement.  Slow shutter speeds have the effect of blurring motion.  Faster shutter speeds have the effect of freezing motion.  In the above photograph, my shutter speed was 1/800sec.  I wanted a fast shutter speed to make sure the kayaker was sharp.  I also wanted to freeze the motion of the water so you could have a sense of the power of the rapids.  The faster the subject moves, the faster your shutter speed needs to be to freeze the motion.