This is a common phenomenon and most of the people have encountered this while moving on ice:falling down! Why does this occur?
According to me, it is due to lack of static friction. But why will it be absent here? What is the cause? I have heard that there is a thin layer of water above the ice . But why,unlike other solids, is this thin layer present? Will it not cool down to ice?
Actually, the surface of the ice is frozen, until you step on it, then it melts. When pressure on the ice is increased, it can melt if it is not too cold. This is why ice skates work so well on the ice. They create a lot of pressure; creating a water layer between the ice and blade. This water layer decreases the friction so the blade slides easily.
From Chemwiki:
... one is able to melt ice simply by applying pressure and not by adding heat.
Walking on ice can can be hazardous if you're wearing a nice pair of dress shoes with slick soles. The backward-sloping phase transition between water and ice cannot explain this phenomenon, and it certainly cannot explain why a hockey puck slides so easily on ice. Nor can frictional heating.
That ice is slippery is instead a boundary layer effect. That the surface of ice was somehow wetted was first proposed by Michael Faraday, but scientists forgot that hypothesis for a while. Scientists didn't have the tools to study this phenomenon until the 1960s, and it remains an area of active research to this day.
The lack of ice molecules above means the water molecules near the boundary layer can't quite organize themselves into a nice crystalline structure. Instead, those boundary layer molecules form a quasi-liquid layer at the ice/air boundary. The thickness of this layer is quite temperature sensitive. This means ice and snow stop being slippery when the temperature gets too cold. Pressure obviously plays a role, too; your dress shoes would make rather lousy ice skates. How pressure plays a role is still a bit up in the air.
The Physics Today article on the subject by Robert Rosenberg, "Why is ice slippery?" Physics Today 58.12 (2005): 50, is highly readable and very interesting. Read it for more info. And don't be afraid to follow the links in my google scholar search.
