Briefly, a fresco is a painting done on wet plaster. Normally done segment by segment, a detailed fresco can be very difficult to create as one must paint quickly and consistently over plaster as it dries.
The Roman's used frescoes to decorate their homes, especially in the villa estates of the more wealthy.
Much of what we know about Roman frescoes (and Roman paintings in general) comes from ancient villas, especially Pompeii, a Roman city near Naples preserved by a volcanic eruption in AD 79, although we have a few examples of earlier Roman and Etruscan room paintings.
Roman frescoes are typically classified into four groups or "styles," ranging from simulation of marble walls to intricate still lifes. You can read more about the four styles on wikipedia here:
Pompeian Styles - Wikipedia, the free encyclopedia
Hope this helps,
Tim
