At high elevations, snow builds up to form glaciers, which flow downhill, extend into warm areas and melt. An 'equilibrium line' separates areas that melt in summer from areas that stay ice covered all year. If more ice melts than accumulates, the glacier retreats. In the past few decades, 400 billion tons of ice, enough to fill 160 million Olympic-size swimming pools, vanish from mountain glaciers around the world every year.
Full article: http://climate.nasa.gov/quizzes/quiz-ice/