18-70 means the focal length covered from 18mm to 70mm, the smaller the value, the wider the lens, the bigger the value, the more telephoto is the lens.
f/3.5-4.5 means the aperture changes from f/3.5 to f/4.5 when you change the focal length from 18mm to 70mm. IF means internal focusing, lens barrel does not moves during focusing. You can learn more about Nikkor lens from www.nikon.com.sg, click on products->Nikkor lenses->all about nikkor lenses