Ice Sheets - the Earth has 2 ice sheets: Antarctica and Greenland. The one in the Antarctic is believed to be about 40 million years old. Sometimes referred to as ice caps, ice sheets form over a large area and then move outward in all directions.

Glaciers - where over time snowfall accumulates, producing large masses of ice which can start to move once they reach a certain thickness. Glaciers occupy approx 10% of the Earth's land surface. Glacier ice crystals have been found as large as a tennis ball.