Ice sheets are the largest accumulation of ice and completely cover the underlying ground. Currently, ice sheets cover Greenland and Antarctica.
Ice sheets are divided into two types; land based ice sheets, where most of the bottom of the ice sheet lies above sea level and marine-based ice sheets, where most of the bottom of the ice sheet lies below sea level.
Ice Sheet, Photo from http://www.ucsc.edu/currents/01-02/01-28/antarctica.html
The ice sheet in Antarctica is the largest single mass of ice on Earth, but it is classed as two ice sheets to distinguish between the two types of ice sheets. The East Antarctic Ice Sheet (10 million square km) is a land based ice sheet while in contrast, West Antarctic Ice Sheet (2 million square km) is a marine-based ice sheet.
Ice sheets are formed over thousands of years by snow and frost crystals slowly being squashed by more snow falling on top. As these crystals are buried the weight of the crystals above slowly squeezes them together and eventually, they turn into glacial ice.