Sheets of Ice
Ice age refers to time periods when large sheets of ice covered large areas of Earth. Over Earth's vast history approximately nine ice ages have occurred with the most recent ice age beginning about 100,000 years ago. It peaked a mere 18,000 years ago. About 1/3 of all land mass was covered in ice. Not only the "Big Apple" (New York), Oregon, Illinois, and Iowa were covered with large sheets of ice, but most of Great Britain and Northern Europe were as well. Glaciers existed on mountains that are free of ice today.
Close to 5% of Earth's water was frozen in the form of glacial ice. This caused the sea level to drop 400 feet (think of a 40-story building to get an idea of the size) and (you guessed it) land that had been covered by water was visible and usable. This new land formed land bridges between the continents which allowed humans and animals to migrate (move from one place to another) to other areas. People came from Asia to North America over the Bering Straits.
When Earth's climate cools for a long period of time (geologically speaking) the ocean cools and contracts and polar ice sheets expand. Water is transferred to land in the form of glaciers and snow. Both of these cause the sea level to drop. Periods of warm climate cause the reverse to happen. The melting continental ice returns to the ocean causing the sea level to rise. Land that was visible and usable during the ice age is reclaimed by the rising ocean.
Glaciers form where climates are cold enough that snow doesn't melt in the summer. The snow is transformed, by recrystallization, into ice. As snow accumulates, the ice thickens at the tops of mountain-valley glaciers or at the domed centers of ice sheets. As the weight of these ice sheets increases, gravity starts to pull them downhill. During periods of stable climate, the size of a glacier remains fairly constant. In other words, as the glacier or ice sheet loses ice due to melting and calving during its gravity-induced (caused) trip to warmer latitudes (ablation zone), it is replenished or supplied with more snow in the accumulation zones (colder latitudes).
Scientists have discovered through geologic records that dramatic changes in climate forced the ice sheets to retreat (toward the polar regions). It has been challenging for scientists to find the causes of such dramatic changes in climate over the past million years or so. "What turns ice ages on and off?" is only one of the questions that scientists are seeking answers to.