The majority of the earth's fresh water is frozen in large sheets of ice known as glaciers. The majority of glaciers on the earth are at present located on Greenland and Antarctica. Glaciers form by the continual accumulation of snow that recrystalizes under its own weight into ice. In the past, as much as 30% of the earth's land surface was covered with glaciers, and these enormous sheets of ice have had a major effect on the earth's landscape.
Glaciers are not stationary. Rather, they flow with gravity from a high point (usually high in the mountains) to a lower point (a valley or the ocean). While their movement is very slow (several centimeters to several meters per year) their effect as they grind over the land is enormous.
Macro Effects Glaciers carve out large areas of land where they pass. These glacial valleys have a characteristic U shaped valleys (compared to the V shaped valleys that result from rivers). Yosemite, the Fjords of Norway and Swiss Alpine Valleys are all examples of glacial valleys. Glacial valleys often have spectacular steep walls and dramatic water falls which occur where a river was shaved in half by the advancing glacier.
When glaciers retreat (or melt), they often leave behind massive scars on the landscape. Moraines are hills of sediment that were pushed out of the way by a glacier. Lateral Moraines form along the sides of glaciers, while Terminal Moraines form at the furthest extent of the glacier.
Micro Effects As glaciers move across the land, bits of rock and debris get stuck in the ice at the bottom of the glacier and are carried along as the glacier advances. The enormous weight of the glacier can force these bits of debris to scrape and polish bedrock that the glacier passes over.
In addition when the glacier melts, it will leave these rocks (sometimes as large as a house) behind, often many kilometers from where they were picked up. Areas which at one time were covered with a glacier often have large boulders in the middle of grassy fields. If you have ever driven through the Sierra Nevada, you may have noticed large boulders sitting atop a bare rock face. These were not placed by Herculean pranksters, but rather by a melting glacier.