As per my last entry, I bought my D5000 with an 18-55mm NIKKOR lens kit. Basically, the unit allows the user to move the lens to a distance away from the camera sensor between 18 and 55mm. This is commonly referred to as a ‘zoom’ lens and can be purchased with different ranges, such as a 55-200mm. There are also lenses that have a fixed range (a prime lens) that are lighter and cheaper than a zoom lens but offer less flexibility.
The distance between the lens and the sensor is called the focal length. Changing the focal length alters the angle of view; the closer the lens to the sensor the wider the angle and vice versa. I drew a quick picture on paint to help illustrate this;
At its shortest, the focal length will capture a lot more of the content of the scene where as a longer focal length will magnify a portion of it and cut out details outside of the frame. Wide angles, for example, are very good for landscapes and architecture where as telescopic narrow angles are good for sport or wildlife photography.
Lenses I use:
Nikkor AF-S DX VR 18-55mm
Nikkor AF-S DX VR 55-200mm