My uncle asked me this very question just a week ago.  I realized it is easy to get to a place where one figures everyone understands the basic fundamentals of wine.  But of course, not everyone does.  So, what is the difference between a Cabernet Sauvignon and a Merlot?  Simply put, the grape.

These terms, Chardonnay and Pinot Noir, don't refer to the company, vineyard, or style necessarily.  There are hundreds of species of grapes and most of these are used to make wines.  The species of the grape is almost always listed on the label (Except jug wines).  Some labels may say things like Meritage, Cuvee, or sometimes Claret.  These are blends of several grapes. 

Also, many foreign wines never put the species of grape on the label, although this is changing.  For instance, Champagne doesn't refer to a grape, but the style and location of the wine.  Champagne is typically a blend of Pinot Noir, Pinot Meunier, and Chardonnay.  A Ribera del Duero is a blend of several different Spanish grape varietals (varieties).