It depends on you wanted to capture the whole image or focus in on the tiny details.
The Focal Length of the lens (measured in millimeters) and the Zoom Ratio determine how much or how little of the scene you are photographing will appear in the final photograph. When you want to show the whole scene, you use a short focal length and when you want to focus on a detail you use a long focal length.
Fixed Focal Length vs. Zoom
A fixed focal length lens cannot zoom in and out on a scene. If you want the subject of your photo to be larger, you have to walk closer. If you want it smaller, you have to walk away. Common fixed focal length lenses are 35mm and 50mm.
Zoom lenses have variable focal lengths. For example, a typical zoom may be able to change from 28mm (wide-angle) to 135mm (zoom or telephoto). The benefit here is that if you cannot walk toward or away from your subject (maybe you're on the edge of a cliff) then the only way you can get closer is by zooming in with your lens.
Zoom and the Real World
Zoom lenses can be habit-forming. It's fun just to stand in one spot and zoom in and out on the scene until you get the composition just right. While many professional photographers will tell you to actually move your body, zoom lenses can be quite convenient.
Zoom lenses are really used for those situations when you can't get close to your subject. The zoom is a good example. Say you want to take a nice photo of a gorilla's head. The gorilla is sitting far away behind a barrier and across a moat. The only way to get the photo that you want is if you have a powerful zoom lens on your camera.
Here are some shots you might want to take, and the zoom that you might need:
You want to take a landscape photograph that shows the whole entire scene. A wide-angle (28mm) camera with any zoom ratio at wide-angle you are not even using the zoom.
You enjoy taking portraits of people. A moderate focal length (100 - 125mm) camera. If you use a wide-angle when taking portraits the facial features of your subject will be distorted. It is much more flattering to use a longer focal length. A 3x zoom ratio is fine.
You want to photograph birds. Select telephoto with full zoom (300 - 600mm). Because of their small size, birds tend to disappear in photos if you are not close enough to them. Minimum requirement of 10x zoom ratio with much better result.