What is a Glacier?

A Glacier is commonly Known as a Frozen river made out of ice and snow. This is the easy way of putting it.
The more difficult way is a mass of land ice, formed by the further recrystallization of firn, flowing continuously from higher to lower elevations.
This term covers all such ice accumulations from the extensive continental glacier to tiny snowdrift glaciers

or

A multiyear surplus accumulation of snowfall in excess of snow melt on land and resulting in a mass of ice at least 0.1 km 2 in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water
As you can see there are many different explanations for the Glacier which may confuse you but i assure you that they are all quite simple.