Ice is the name given to water (H2O) in the solid state.
The point at which water changes from a liquid into ice is 0oC.
Ice is an unusual solid, because it is actually less dense than the liquid it changes from. This is because the water molecules pack themselves in a certain way in three dimensions.
Each one of them has polarity, giving them a boomerang-like shape, with a slightly negatively-charged middle, around the oxygen atom, and two slightly positive ends, around the hydrogen atoms, which want to stay apart from the similar regions of nearby molecules, whilst being attracted to the regions of opposite charge.
In the liquid state, the polarity and the bent shape are still there, but the kinetic energy of the molecules is higher, so making and breaking of these weak polar attractions (called hydrogen bonds) goes on without stopping the free movement of the molecules. In ice, though, the presence of hydrogen bonds determines the arrangement of the molecules.