When someone says "glacier," most people imagine a huge mountain of ice. While this can be true, it's not the only form a glacier takes; according to Dr. Osborn, any large, natural ice accumulation of land ice can be classified as a glacier.
Glacial ice is different from the ice in your freezer -- it has more density and flexibility and is created from snow in a constant cycle of melting and refreezing. Eventually, after many trips through this cycle, the snow becomes "firn." Firn is much more compact that seasonal snows to which most people are accustomed and, over the course of a century, multiple layers of firn accumulate until it hits a threshold of 50 meters -- at which point it becomes glacial ice.
Ice sheets are a kind of glacier characterized by extremely deep and wide masses now found only in the polar regions. These expand during ice ages and cover a larger and larger portion of the planet -- in the depths of an ice age, ice sheets can cover up to one third of the Earth's surface as opposed to the 10 percent of land they cover today.
Like other forces of nature, such as wind and water, ice sheets serve as an erosive that, once gone, alter the land they once covered. Troughs created by ice sheet movement often become fjords and lakes. Ice sheets also leave behind deposits of glacial drift -- debris frozen within the glacier than escapes as it moves on or melts -- creating till plains of fine sediment and moraines of soil and rock.