Glaciers develop over many years in places where snow has fallen but not melted. Snow is compacted and turns to ice. The weight of the ice means that it starts to slip down mountain sides over time.
A glacier is a system. There is a zone of accumulation where snow is added. This is normally at the start of a glacier in a highland area. As more and more snow falls, it is compacted so the bottom layers become ice.
Ice moves downhill due to the force of gravity.
Near the end, or snout, of the glacier ice may melt. This is the zone of ablation and is more likely to occur in warm summer months.