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ok there should be a number coupled with the "f" this refers to the f-stop of the lens, this is often more important to a lens' value than the mm. F-stop refers to the maximum aperature (the hole that lets light through the lens) that the lens is capable of. A lower number for f-stop (1.4,1.8,2.8,3.5,etc.) means that the lens is able to take pictures in lower light situations. This is often spoken of as the "speed" of the lens because aperature (f-stops) and shutter speeds are mathematically equivalent. One f-stop equals one shutter speed increment. A lower f-rating is better.
mm refers to the shape or strength of the lens and depends on the size of the film or type of sensor in the case of digital cameras. For 35mm film an 18mm will be quite wide-angled with a fair amount of distortion on the periphery of the image. 50-70mm is a standard focal length for 35mm film corresponding to how your eye generally sees things. Beyond 70mm the lens will have a telescopic effect, bringing things closer.
If you're having trouble picking between these two lenses I would ask what kind of photography you plan to use it for. Assuming both have the same f-rating, the 18-70 lens will be better for wider landscape photos or in close quarters, the 55-200 lens will be better for taking wildlife or other instances where you will want to zoom in on a specific feature from a distance. But 55mm is fairly wide so either lenses could work as an all-arounder. I would prefer the 18-70 lens, it probably has a wider aperature (lower f-stop) and suits the type of photography that I do mostly. It's a question of preference.
In the case of digital SLRs, the lens sizes correspond typically to a 35mm film format.
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