The 'Creative Zone' on your Camera allows the user to have more control over the shot. If up until now you have been using the 'Basic Zone' you may have noticed that there are very few (if any) settings that you have control over.
There are five modes in the Creative Zone. This basic tutorial only covers the Av mode (Aperture-priority) and the Tv mode (Shutter-priority). For tutorials on the other modes please look at the advanced tutorials on the Creative Zones here.
Take for example this Scarlet Tiger moth. When I took the picture on the left I selected a narrow aperture (high f-number) and the camera decided on a 1 second exposure. When I took the picture on the right I selected a wide aperture (low f-number) and the camera decided on a 1/30 second exposure. It is easy to see from these two pictures the effect aperture has on the final shot: The picture on the left with the narrow aperture (high f-number) has much more depth of field than the picture on the right with the wider aperture (low f-number). In the left picture the plants in the background are not fully in focus but are still identifiable as plants. The picture on the right with the wide aperture as much less depth of field resulting in the background being blurred. The picture on the right really draws the viewer's eye towards the moth; the moth is the focus of the picture. In the left picture the eye is drawn away from the moth; the moth just becomes a part of the picture as opposed to being the picture.
Setting your camera to Tv mode puts it into shutter-priority. What does that mean? That means the camera lets you choose the shutter speed and it selects the appropriate aperture for the correct exposure.
A fast shutter speed is necessary when you want to freeze movement; e.g. sport shots. Slow shutter speeds have their own uses. If you are shooting a subject when there is not much light then a longer exposure can help you catch all the detail; e.g. shooting at dusk or taking pictures of stars. Another popular use for a longer shutter speed is for taking pictures of water. A longer shutter speed can give a nice 'feathery' effect to the water.
The pictures above illustrate the effect of shutter speed on a moving subject. The faster shutter speed of 1/200 second has captured individual drops of water. The slower shutter speed of 1/15 second has not frozen the movement resulting in a blurred subject; however when the subject is water this can be a desirable effect.