The Ice Ages
Over the history of our planet, there have been five (or maybe six) major ice ages. There is even a theory being seriously debated called Snowball Earth, which proposes that the planet was engulfed in ice sheets. For our purposes, we only need to be concerned about relatively recent events.
The Quaternary period starts about 2.58 million years ago and is ongoing. We are in an interglacial period, the Holocene, with only the Greenland and Antarctic ice sheets remaining.