The usual material is dendix which is similar to a short haired brush with the bristles sticking upwards. This is arranged in a hexagonal pattern of approximately 1 inch strips of bristles in a 4 inch hexagon. It doesn't really feel that much like snow but it is good enough to ride a snowboard or ski on with some practice. Water is often sprayed onto the surface of the dendix to lubricate it and increase the skiing speed.
More recently a new material, SnowFlex, has been introduced. The new material looks like white AstroTurf. This is better than dendix but still not as good as snow.