In general, the Earth seems to have been ice free even in high latitudes except during relatively rare glacial maximums such as the one from which we emerged from 10 to 15 thousand years ago.
The causes of glaciations have been much debated ever since the phenomenon was clearly identified in the 17th century. Modern theories tend to revolve around periodic oscillations in the Earth's orbit; hypothecised periodic changes in solar output; and/or the affects of continental masses drifting into polar regions where Antarctica currently resides.
Known periods of glaciation include the Huronian (2400Ma-2100Ma), Stuartian-Varangian (950Ma-570Ma), Andean-Saharan (450Ma-420Ma), Karoo (360Ma-260Ma), Cenozoic (30Ma-Present). Not every year in each interval was a time of complete or even partial glaciation. The best studied glaciation -- that of the recent past appears to have taken place in at least four separate ice incursions and retreats. Unfortunately the scouring action of each glaciation tends to remove most of the evidence of prior icesheets almost completely except in regions where the later sheet doesn't achieve full coverage. It is probable that glacial periods other than those above have been overlooked because of the paucity of exposed rocks from high latitudes from older time periods. The Varanger glaciation was especially severe and may have extended to the Equator. This has lead to a recent Snowball Earth hypothesis that the Earth froze over completely in the late Proterozoic then thawed very rapidly as trapped water and carbon dioxide were returned to atmosphere.
An ice age is sometimes used to refer to a single glaciation, or to an entire period of repeated glaciations such as the recent 30 million years of the Cenozoic period.
