Wikipedia defines art as "the product or process of deliberately arranging items (often with symbolic significance) in a way that influences and affects one or more of the senses, emotions, and intellect." Humans have been creating art for tens of thousands of years. The earliest object defined as art dates from around 75,000 years ago. While the earliest artists carved and painted their works on caves or sea shells, contemporary artists have a diverse range of mediums to choose from. Some artists use paints, others work with clay while others do their creating on computers using programs like Photoshop and Illustrator. (In a recent computer science class I took, we had to create our own works of art using Java code.)
But that last example wasn't truly created by computers; they merely facilitated its creation. Humans are still solidly superior to computers when it involves the actual creation of the work. A computer-generated piece of art is one that is wholly developed and conceived by artificial intelligence. Researchers like Ray Kurzweil are doing amazing work with artificial intelligence that creates art, but it will be a long time, if at all, before computers are able to produce works with as much emotional and technical depth as a human.