So to the point.
More roman paintings than what anyone could imagine existed showed up when the Campanian remains were rediscovered during the 18th century – more or less every wall in the city had some sort of painted decoration. This massive amount of paintings enabled a German scholar, August Mau, to create a system where the paintings could be categorized and dated, as he noticed some fundamental differences. This system is now somewhat questioned but it is still useful as a frame of reference.
The first style
The first style is dated to the 2nd century Bc and it’s recognized by the simple look. The intention was to imitate marble blocks by using painted plaster that was polished. This first style wasn’t very common in Campania in 79 ad but we find it in some of the older (obviously conservative) upper class houses.
The fauces (entrance) of Casa del Fauno (Pompeii). Notice the square painted blocks under the small columns.
The east side of Via di Mercurio in Pompeii. It was probably the owners of Casa dei Dioscuri that paid for this outer wall - there's unfortunately no color traces left but it's a fair guess that this wall once looked like shining marble.
The second style
The second style is found from the early 1st century Bc and onwards. Here we find distinct three dimensional motifs - there are however still something left of the 1st style as the imitated marble blocks sometimes are kept as frames and visual support.
Here we see how the characteristic three dimensional feature in the second style may look.
Notice how the painting here is surrounded by red blocks. This is however probably a rather late second style painting as it has elements that remind us of the third style.
We also have a wonderful example of this style from the House of Augustus at the Palatine.
The third style
This is probably the second easiest style to recognize and it dates from the Augustan era, i.e. 30 Bc and onwards. The third style is in a way a development that came from the second. We still see same architectural details as in the second style but they are now tiny and used to frame small central motifs.
A wall from Herculaneum painted in the third style.
Detail of the architectural elements.
A central motifs. These are normally, as here, small dream landscapes or small animals such as swans and griffins.
The fourth style.
It is rather easy to take the fourth style for the third and the other way around. The style is to a certain degree a combination of the second and third. The paintings we find in this category normally dates from 50 Ad and onwards.
Here we see a fine example of the 4th style. It is easiest recognized by the still tiny architectural elements in combination with this almost insane way to depict buildings and figures.