Very simply, the subject matter is what is literally represented in the work of art. The subject matter is the thing that you see before you when you look at a painting, a sculpture, or another piece of art. For many people, the first and most lasting response may be to a work of art's subject matter. The subject can be usually classified into some very broad categories. These categories are not a definitive list.
Many works of art can fit into several categories at once.
A self-portrait is a portrait of the artist done by them self. On the right is Houses at Auvers by Vincent van Gogh. They are both landscape paintings even though one is of a city and the other of the country. Pigeons could also be identified as a genre painting. It shows a view out of the artist's window of people raising pigeons on the rooftops in New York City.