Skeuomorph is the exact opposite of flat design. It involves the process of making all icons as though they have been picked from the real life. This is the essence of skeuomorphic design. The design concerns with the application of shade, shadow, gradient color effect, textures to the icons, forms and buttons concerned.
Though you can barely see the use of skeuomorph, it was quite popular back in the days. An example would be the design of the older apple iOS.