A 200mm lens is in the category of lenses called "fixed focal length" or "prime", which cannot zoom in and out. The more common lens type is the zoom, which does allow you to zoom in and out.

The primary tradeoffs between prime and zoom lenses are:

• Aperture - Typically, prime lenses tend to operate at wider maximum aperture than zooms. For instance, Nikon makes 50mm, 85mm, and 105mm prime lenses that can expose down to an aperture of 1.4, whereas the best zoom lenses that cover that range can only focus down to 2.8. In terms of the photo, that can be a noticeable difference in the Depth of Field, which controls how much of the image is in focus and how much is in blurred out.
• Optical quality - Prime lenses are almost universally better optically than equivalent zooms. They achieve better sharpness by usign fewer optical elements. You can think of this as fewer pieces of glass between you and your subject, thereby reducing the amount of distortion, letting more light through, and having more fine-tuned control of the final image.
• weight - Prime lenses are always much lighter in weight than their zoom counterparts

You asked specifically about the 200mm lens. This lens is often used for:

• Wildlife - When you are on safari, or birding, your subject can be as far as 50 to a few hundred yards away, and if you still want to fill the frame of your image with that subject you need a hefty magnification. The 200mm helps accomplish that.
• Sports - Same as in wildlife, if you are standing on the sidelines of a sports game, you need good magnification to capture the action up close in your frame.
• Portraiture - This lens is also useful for portraiture, but it requires standing very far from your subject. Some professionals will do this to achieve a very dramatic effect and a maximal blurring of the background.

This lens is best used with a tripod or monopod because when you are using that much magnification, even the slightest bit of camera shake will significantly impact your image.