The focal length of the lens is the number given in millimeters. It basically describes the field of view of the lens and its magnification. Most folks think of shorter lenses as wide angle and longer lenses as telephoto, and certain subjects tend to lend themselves to one type of lens vs. another. (See: the Nikon lens simulator).

When there are two numbers that means the lens is a zoom lens, where you can vary the focal length over a range. The first number is the wide end of the range, while the second number is the telephoto end of the range.

The price of a lens tends to increase as you go farther towards the extremes of focal length. Very wide and very long lenses tend to require a lot more glass and possibly a lot more corrections to get good image quality than a lens in the midrange. You also need to know that a lens is typically described by TWO numbers: the focal length and the maximum aperture (given as an f-number). Two lenses that have the same focal length(s) may have differing maximum apertures, or very different optical designs to achieve different goals (see: How do I choose a zoom lens for my Canon DSLR when I'm ready to move beyond the kit lens?)

See also: What do all those cryptic number and letter codes in a lens name mean?