Glaciers usually begin between mountains. Snow builds up between the mountains over thousands of years. It compresses into a large thick ice mass. Glaciers contain the largest amount of freshwater on Earth. Glaciers are constantly moving. Gravity makes them flow downhill.
"Baltoro glacier from air" by Guilhem Vellut from Paris - Glacier. Licensed under CC BY-SA 2.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Baltoro_glacier_from_air.jpg#/media/File:Baltoro_glacier_from_air.jpg