Roman-period wall paintings had been found before the excavations at Pompeii and Herculaneum, but often they were fragmentary and from unknown locations. With the discovery of so many well-preserved paintings in the Vesuvian region, it was possible to compare the paintings as a group and classify their styles of artistic composition.
The recognized styles of Roman painting are:

The first style, in use from the 3rd century BC to the 1st century BC, mimics stone blocks or polychrome marble. Sometimes a painted garland might also be incorporated into the painted decoration.

The second style, used from the beginning of the 1st century BC up to the 20s BC, is noted for representations of architecture painted in perspective, on various levels, and often incorporating distant landscape views. In some instances, near life-size images of people are depicted within the architectural framework.
In the third style, in use from the 20s BC to the middle of the first century AD, no attempt is made to fool the eye with foreshortened views and illusory openings. The decoration is divided into panels, usually taking the form of a central panel that is often filled with a large representation of a mythological subject, and which is framed by refined decorative motifs.
The fourth style, still in use by 79 AD, combines elements from the third style
– with greater diffusion of ‘pictures’ in the centre of the panels – with fantastical architecture reminiscent ofthe second style.


In addition to the ‘styles’, there are two other types of subject matter:
garden painting – in which whole walls are frescoed with luxuriant gardens, decorated by fountains, sculptures, and birds, and popular painting which reproduces scenes of daily life, divinities and religious ceremonies, and historical episodes.
Source: Soprintendenza archeologica di Pompei