#1. "RE: Lens for Nikon D3000" In response to Reply # 0
The aperture is a mechanism in the lens that varies the light entering through the lens. The f/stop numbers designate the size of the opening and are a ratio of the focal length and aperture size. A 200mm lens at f/5.6 lets in the same amount of light as an 18mm lens at f/5.6.
Your lenses have a variable maximum aperture depending on the focal length at which they are set. For example, the 18-55mm has a maximum f/stop of 3.5 when at 18mm and 5.6 when at 55mm. The aperture can be adjusted anywhere from the maximum to the minimum (f/22). Smaller numbers, like f/3.5, are larger openings and let in more light than larger number f/stops like f/16. The aperture, along with shutter speed, determine the amount of light striking the sensor, i.e. exposure.
18mm is a moderate wide angle, meaning you get a fairly wide view of the scene in front of you. 200mm is a medium telephoto which has a narrower view. They're good for any thing you want to shoot!