A common way of describing a definition is to delimit its genus (the general class of things to which it belongs) and differentia (how this particular class of things differs from the others of its genus). The genus of art is "works of expression". This includes a broad variety of expressive things such as journalism, ordinary speech, temper tantrums, clothing styles, technical manuals, and of course art. The identifying characteristic of expressive works (of all kinds, not just art) is that they convey information by the intentional manipulation of a medium (as opposed to the way we discover information by just observing things directly which we might call "observation" rather than expression).
Art comes in several types, most commonly distinguished by the medium in which the artist does his work. Art made by the creations of visual images on a surface can be created by painting or drawing. Sculpture is art created by the manipulation of three dimensional shapes in some medium like marble, bronze, glass, clay, or plastic. Music is art created by the manipulation of sound, generating patterns of things like melody, harmony, rhythm, and instrumentation. Literature (including poetry) is art created through the manipulation of words. Dance is art created through the motions of the human body. Drama (such as plays, movies, and opera) is a composite art form in which a variety of media (sound, words, sets, etc.) are combined. They all have in common the manipulation of a medium in order to create works that express some idea, but they have some qualities in common with one another that they do not share with other forms of expression.
How does art differ from these other kinds of expression? In other words, what is it that painting, sculpture, music, literature, and drama have in common with one another which is not in common with other kinds of expression? The difference between these classes has to do with the method of expression (as opposed to the content or the medium of the expression). Specifically, the way that art accomplishes its expression is through the manipulation of a medium as a selective recreation of some aspect of reality. That is to say that the artist "fictionalizes" reality in order to highlight some idea he thinks is important, and to diminish ones he considers irrelevant to his intended message. The artist's message is paramount in this selection rather than a slavish devotion to describing the concrete state of affairs. This doesn't mean that an artist must not or should not present his subject through use of a highly realistic style (though of course he can do that if he wishes since realistic styles in some media offer powerful advantages). The artist selects patterns of design or style according to the intended meaning and if he's a good one, based on an understanding of the strengths and weaknesses of his medium and the craft of manipulating it. There are millions of possible styles, that work and millions more possible subjects and subtleties of meaning, and trillions of possible combinations of these, let alone the vast numbers of possible combinations of technique and detail. Despite this vast scope of possible subjects, designs, compositions, and styles, the number of possibilities is not infinite. Some things do not fit this definition, and not all accomplish a given goal as well as others.
One common notion (spread throughout society in the 20th century) is that art cannot be defined, cannot be "limited" by definitions, and that anything that "expands the definition of art" is good. My perspective is directly contrary to this view, and I should point out that nobody has ever demonstrated that these strange notions about the definition of art are correct (or really even proposed a coherent argument to that effect). They have just repeated them billions of times with an air of authority that has beguiled many people into believing some very strange things.
