Attempting to define realism or to clarify the various styles of representational painting can be a challenging task. When writing about today's painters, I always hesitate to use words like "classical," "realist," "contemporary," "traditional," or "modern" because they do not always fully capture what I'm trying to describe. For example, a word like "classical" by definition refers to Greco-Roman or Renaissance ideals, but by connotation has come to mean an art form that embodies a certain timelessness and order. "Contemporary" undoubtedly means anything taking place in our time, so by that definition any painter alive is contemporary. But critically speaking, I would not call all painters today contemporary--I'm more apt to use that distinction when a painting has a more modern or conceptual feel.
Artists themselves have felt the confusion and in an attempt to bring clarity have adopted such labels as new realists, figurative realists, classical realists, contemporary realists, and so on. These phrases have found their meaning in a Post-Modern context, which generally defines what it is by comparing itself to what it is not. The New Realists of the early 1960s, for example, made it clear that they were not your grandmother's realists reveling in Caravaggio and Jacques Louis David but rather were trying to bring elements of representation to the Abstract Expressionist aesthetic. The Classical Realists of the early 1980s led by Richard Lack were differentiating their brand of realism from other forms of representational art of the time, even though they knew that within the context of art history "classical realism" was a contradiction in terms.
Realism can mean different things to different people and has changed meaning over time, which is another reason why it's difficult to define. Most would agree that realist painters are recognized by their choice of subject matter. Like Gustave Courbet--by most accounts considered the first Realist--or any of the Russian painters throughout history, a realist paints the real world and finds beauty and interest in everyday people, places, and things that the rest of society might find mundane. The realist is also defined by his or her technical execution, which aims for an accurate, truthful representation of the subject. The stylistic differentiation gets tricky among artists, however, because many realists who work from life do not want to be put in the same category as realists who work from photographs or in a photorealistic manner. This confusion translates to the public, who will often comment that a painting looks like a photograph, as if that were the highest compliment one could pay an artist.
As you can see, the parameters surrounding realism are rather ambiguous. I think this is because realism is redefining itself as we speak and also because a new movement is on the horizon. I believe that movement will embody a language that is not yet in our artistic vocabulary, forcing us to find new words and a new criterion.