Water molecules interact with other water molecules, and are generally attracted to each other. By interacting with each other, they become more stable. However, above the freezing point of water, there is enough energy (heat) to cause the water molecules to vibrate, rotate, and move around. All this energy prevents water molecules from freezing. However, as the temperature decreases (and the energy of the system decreases), the molecules move around less and eventually they are still enough that they can interact with each other, and form a solid.

Think of it like a group of people trying to hold hands. If these people have a lot of energy- -they're running around and flailing their arms-- it's difficult for them to all hold hands. However, if the people have less energy--they calm down and stop moving--people next to each other will be able to easily grab the arms or hands of their neighbours.

Water molecules are what is called polar - they have a minus pole where the oxygen is and a plus pole where the hydrogens are. Plus attracts minus, so water molecules form long chains with the plus pole of one molecule attached to the minus pole of another.

Temperature is the motion of molecules. The hotter they are, the more they are moving. When water is hot, the force that attracts plus to minus is too weak to overcome the existing motion, but when the water is cold, the force of plus attracted to minus is strong enough to hold the molecules in place. This is what a crystal is, and when something makes a crystal, we call it freezing.