In art, style refers to characteristics within a group of works that have commonality. If one sees three or four paintings by Vincent van Gogh, his unique style is visually discernible. Then, when another work is seen, one can readily say that it is either a work by van Gogh or by someone who is trying to imitate his style.
A work can be placed in abroad category or a specific category, and one work can exist in several categories. For instance, Expressionism is a broader style and Der Blaue Reiter is a more specific style. But, Der Blue Reiter is an Expressionistic style - a subset of Expressionism. Pablo Picasso has an overall style that makes his works identifiable, and yet he created works in Pointillism, Cubism, Surrealism, as well as his 'Blue Period' and his 'Rose Period'.
Style is not restricted to paintings. Once an artist or a more general style is made clear, then whether it is in music, architecture, sculpture, photography, or any other discipline other similar works are easily discernible.