First off, a camera lens worth its salt is rarely in fact just one lens. We call it “a lens” out of habit, but in most cases it is in fact a conglomeration of many lenses, referred to most commonly as lens elements. “Lens elements” as a phrase is itself misleading. In fact, the double meaning of the term "lens," referring to a camera part, and "optical lens," referring to any lens used for a variety of purposes, lends itself to a kind of linguistic and lexical problem: when referring to the parts that make up the whole, both have the same name. Suffice it to say that most camera lenses are not lenses at all in the most traditional sense, but rather are themselves a series of optical lenses collected together to make a more complex and precise device.
Why are there multiple lenses in digital camera lenses? The simple answer is that a single lens has what is termed “aberrations.” These are areas of the lens where light becomes distorted, either losing focus or distorting coloration, and in fact a host of other issues. The multiple lenses serve to correct these aberrations.