Zoom range of digital lenses

[WillyNelson]

I was just reading up on the Nikon 50D and came across this in the blurb about a lens:

With a practical wide-angle to telephoto zoom range of 18-70mm, this Nikkor lens offers the a 35mm format equivalent of a 27-105mm lens.

Can someone explain that to me please. How does an 18-70mm lens have the equivalent of up to 105mm?

 

[WillyNelson]

That's what I was wondering. So it doesn't mean you get extra focal length, just a tighter image.