The thickness of this layer depends on the temperature. It is this liquid layer that makes the ice slippery. The water molecules can be seen as marbles, densely thrown on a floor. Or consider how a thin water layer on an asphalt road can result in aquaplaning, the cars sliding across the road without any possibility of control. It is also because of this layer that you can make snowballs.
Snow is simply special forms of ice crystals.
Which type that will be formed depends mainly on the temperature and supersaturation level in the cloud where the formation takes place. It all starts with a cold cloud and a condensation nucleus, a microscopically small particle which can act as a starting point for the ice crystal formation. This particle can consist of soil, volcanic ash, dust or even be of extraterrestrial origin. It is also possible to artificially initiate the condensation by sowing the cloud with silver iodide crystals. Around the condensation nucleus a hexagonal ice plate is formed.