Paintings can be grouped by certain characteristics. These characteristics usually come about when painters themselves either become disillusioned with what has gone before, see the world in a different manner, or otherwise change their view of what they are trying to express.
By changing the ways in which they paint, apply colour, texture, perspective or the way they see shapes and ideas the artist lays down a different set of "rules". Over a short period of time, if other painters see the rules as being valid for themselves they will also pick up the characteristics. The works then take on a similar form or "style". The artists themselves may give the style a name - "Expressionist" for example, or the style may be given a name such as "Abstract". The name now determines the style of painting.