When we talk about wall paintings or mural paintings, most of us call them ‘frescoes’. This might not be technically correct for all the murals.
Technically, if the painting is created ( or we can say that the colours are applied) on a plaster which is still wet, the painting is called fresco (Italian word which means ‘fresh’). While painting a fresco, no binder is required to be added to the mineral pigments that are applied because the chemical reaction of the wet plaster with the carbon dioxide in the air creates a hard layer of calcium carbonate and the pigments are securely fixed in the calcium carbonate layer while drying. This layer of colours/pigments is so secure that it is not affected by dampness/or moisture etc. Such paintings are called buon fresco. But, many a times complete paintings are not done in fresco buon technique, in such cases, the base layers of colors are done on fresh plaster (fresco buon), but the details and some outlines are painted later when the plaster has dried up and hence with the binder is mixed for good adhesion. These details are not well protected from the damages of moisture and vegetation because of dampness. These superficial/upper layers or details are in fresco secco.
Tempera paintings require a binder to be added to the pigment colours to make them stick to the plaster. They are also quite stable, but susceptible to damage by moisture and dampness. Tempera paintings are done on dry plaster, so there is no stress of the time deadline.