In order to recreate the image you see when you look through the viewfinder, a camera lens directs light through curved pieces of glass to the digital sensor or the film in the camera. Lenses are composed of several elements that attempt to relay the light rays to the sensor as accurately as possible. The lens also uses those elements to focus on the subject and give you a sharp image.
For action photography, you'll need a zoom lens. This enables you to change your focal length -- the ability to focus on an object -- in an instant. This is important in action photography, where the subjects don't stand around and pose as if they were in a portrait studio.
There are two types of lenses: wide angle and telephoto. For action photography, you'll need to stick with the telephoto lens. Its longer focal length can focus on objects that are far away and allows you to situate yourself some distance from your subject and still get a nice shot.
When purchasing a lens for action photography, you should understand the aperture range of the lens and how this will affect your image. Aperture range refers to the amount how wide the aperture can close or open up to let in light. A specific number, known as an f-stop, describes how wide the aperture is at any given moment. It may seem confusing, but the smaller the number, the larger the opening. An f-stop of f/1.4, for instance, will have a wider opening than an f-stop of f/22. The more light you let in, the faster the camera's shutter speed can be (and vice versa). Fast shutter speeds are crucial when shooting action. Users of DSLR cameras will find that lenses that let in more light provide brighter images on the viewfinder, which is helpful if you're shooting in low light.
Your need for a telephoto lens also means that you'll need a camera with optical zoom rather than digital zoom. Optical zoom physically extends the lens to get closer to and magnify the subject. Digital zoom simply crops the photo inside the camera and enlarges it. The enlargement process creates additional pixels in the image, a process called interpolation. In order to enlarge the image, the camera is imagining pixels based on the pixels that are actually in the photograph, but these imagined pixels often aren't accurate and tend to distort the image. Focal length and aperture specifications are listed on the lens box.
Now that you know how a lens harnesses the light to create an image, keep reading to find out how flashes help create light in action photography situations.