Glaciers are bodies of ice, firn, and snow, built up over time when accumulation of snow exceeds ablation (i.e. melting) over a period of many years. Glaciers make up 10% of the world’s total land area, with the majority of global glacier ice found in the vast ice sheets of Antarctica and Greenland.
They are the largest source of fresh water on Earth, and as such mountain glaciers are an important water source for a large portion of the world's population, notably in the Indian subcontinent and South America.
Glaciers are sensitive to long-term changes in the Earth's climate and global temperatures, and are a major source of global sea-level change.
Sea level has been rising little by little over the past century. Part of the sea level rise is due to breaking of icebergs and melting of our world’s land ice. The most important and major ice sheets are in Greenland and Antarctica, because they contain about 75% of our world’s fresh water. 
What happens when glaciers melt
Ten percent of our land is covered in ice all year around. All that ice is made out of a large volume of water which if it melts it will end up in our oceans. This means our oceans' water level would rise. Once pieces of ice break and fall into the water, the heat is transferred to the ice more quickly than it is through air, which makes the ice melt faster. An example is putting ice cubes in a glass of water and seeing how the level of the water goes up and it stays at the same level because the ice is in the water. When the ice on land melts it flows in the water rising its levels more. 
How glaciers move
Glaciers are known as “rivers of ice” and actually move. The cause of the motion of glaciers is gravity. A glacier molds itself to the land and also sculpts the land as it slides down a valley. Glaciers slide on their beds, which makes them able to move faster. If a glacier looses more of its volume than it gains as it moves, the glacier will shrink in size. 
Why this matters
Reducing the amount of human activities such as using gasoline for cars and coal burned for electricity can help slow the melting process of the ice. If the sea level rises due to the melting of glaciers it will affect humans and other coastal residents. NASA’s research has shown that by 2050 about $66 - $106 billion worth of property is probable to sit below sea level. If nothing is done to slow it down the land will be in trouble and will suffer the consequences. 
Other closely related articles in this wiki include:
- Field Notes From a Catastrophe by Elizabeth Kolbert
- Climate Kids, NASA's Eyes on Earth.
- Michael Mann, Confronting the Climate Change Challenge - Conversations at Penn State.
- Climate Change Documentary.
- Richard Alley's Climate Change Future - Conversations from Penn State