There have been four major periods of glaciation in the Earth's past. The first, and possibly most severe, occurred from 800 to 600 million years ago (the late Proterozoic Age). It has been suggested that the end of this cold period was responsible for the subsequent Cambrian Explosion, though this theory is recent and controversial. A minor series of glaciations occurred from 460 to 430 million years ago. Two more extensive glaciations were from 350 million years before present to 250 million, and from 4 million years ago to about 10,000 years ago (the Pleistocene period).
In between, there are multi-million year periods of more temperate climate, but also within these abovementioned periods (or at least within the last one), temperate and severe periods occur. The colder periods are called 'glacial periods', the warmer periods 'interglacials'. Often the term 'ice age' is also used for 'glacial', but in this article we will not do so to avoid confusion.
In fact, it seems likely that we are in an interglacial now, the last retreat ending only 10,000 years ago. There has been some fear of a new glacial starting, but the recent global warming has erased those, and the opposite is more feared now.
The cause of ice ages remain controversial, but the general consensus is that it is a combination of up to three different factors: atmospheric composition (particularly the fraction of CO2 and methane), changes in the Earth's orbit around the Sun (and possibly the Sun's orbit around the galaxy), and the arrangement of the continents.
The first of these three factors is probably responsible for much of the change, especially for the first ice age. The "Snowball Earth" hypothesis maintains that the severe freezing in the late Proterozoic was both caused and ended by changes in CO2 levels in the atmosphere. However, the other two factors do matter.
For reasons that are unclear, an abundance of land in the arctic and antarctic circles appears to be a necessity for an ice age. The Earth's orbit does not have a great effect on the long term causation of ice ages, but does seem to dictate the pattern of multiple freezings and thawings that take place within each. The complex pattern of changes in Earth orbit and the change of albedo dictate the occurrence of glacial and interglacial phases - this was first explained by the theory of Milutin Milankovic which is confirmed by the recently discovered details about the last ice age.
The last glacial and interglacial phases of the Pleistocene are named, from most recent to most distant, as follows (names before the '/' are North America, names after it Northern European, dates in thousand years BCE - note that in Eastern Europe and the Alps yet other names are used):
The Wisconsinan glaciation has had a considerable effect on the landscape of the Northern Hemisphere. The Great Lakes were carved out of the Canadian Shield by ice, Long Island was formed from glacial till, and indeed the watersheds of Canada were so severely disrupted that they are still sorting themselves out -- the plethora of lakes in northern Canada can be almost entirely attributed to the action of the ice. Before the theory of ice ages, such catastrophic changes were usually attributed to flood.
The end of the last glaciation also corresponds quite closely to the development of permanent human settlements and agriculture, and it is possible that there is a connection between the two events.
See also Little Ice Age.