One type of glacier is a valley glacier. Snow builds up on high mountain valleys. As the snow builds up, it becomes compacted into ice. The valley acts like a river bed, and the ice become the river. Just like everything else, gravity works on this river of ice and snow and causes it to move slowly downward.
Another type of glacier is a continental glacier. In some areas, like the polar regions, it's so cold that all precipitation falls as snow. One can only image the force of that snow that has been building up for thousands of years. As the weight increases, the pressure increases, causing the snow to form into ice. As the ice forms, it moves outward in all directions. In some places, the ice reaches the sea and breaks off into massive floating chunks of ice.
