Sleet is small, partly melted bits of ice. It is formed either by raindrops falling through an air layer that is below freezing or a snowflake passing through an air layer that is warmer and melts the snowflake. Hail is small bits of ice or ice and snow that have a soft center. As the hailstones bounce up and down in the atmosphere, they form alternating layers of harder and softer ice or snow until some become quite big, though most are small. Snow is formed by water vapor being frozen about a dust particle in the air, forming crystals of hexagonal shape.
Rain is water vapor that forms on a dust particle that becomes heavy enough to fall from the sky.
Rain and snow amounts are measured with a rain gauge. A rain gauge is a special container marked evenly so that the amount that falls in a given time period in a location may be measured.