Glaciers are produced by the slow accumulation of snow over many years. As it accumulates, pressure packs it into a large ice mass. Because of the mass, glaciers have the ability to slowly move, much like a river, but much more slowly.
Glaciers cover about 10% of the Earth's land mass, down from around 30% during the last Ice Age. However, due to climate change and rising global temperatures, glaciers across the globe are very quickly shrinking and disappearing over a period of just a couple of decades. Soon glaciers will be few and far between on the Earth. Places like National Glacier Park in Montana may be glacier free.
If you've never seen a glacier, do so very soon. If you wait, it will be much harder to easily reach one.