What exactly are differences between frescos and icons?

Answer 1

An icon is a religious representation. A fresco is a painting done by a particular method---usually mixing paint with wet plaster. Fresco painting was frequently used in churches to create large religious scenes. An icon may BE a fresco, or vice versa. But a religious icon could also be cast in metal, carved in stone, embroidered on cloth, done in mosaic work, printed on paper or metal, etc. creating free-standing, three-dimensional sculptures.

Answer 2

A fresco is the art or technique of painting on a moist, plaster surface with colors ground up in water or a limewater mixture and the painting that results from it. An icon (in the Eastern Church) is a representation of some sacred personage, such as Christ or a saint or angel, painted usually on a wood surface and venerated itself as sacred. An icon can also be generically an image or picture.