How does a glacier erode a landscape to form a U-Shaped valley?

A u-shaped valley starts life at high altitude near the peaks of a mountain range. As snow falls, it gradually compacts due to pressure and turns into ice. As this ice gets heavier and heavier, it becomes a solid and, due to gravity, it begins to flow downhill. As it flows it follows the easiest course down which is usually a river valley. River valleys tend to be v-shaped but, due to the material carried by the glacier, the ice erodes the sides of the valley and turns them from a 'u' into a 'v' shape. 

Glaciers can grow to be over a kilometre tall and over 100 kilometres long so it is typical to see U-shaped valleys that have cliffs in excess of 100 metres and that extend for huge distances. A feature that will tell you that a landscape has been glaciated is something called an 'irratic' these are huge boulders caried by the glacier and dropped onto the landscape as the glaciers loose energy. So, if you see a big rock that looks out of place, you can be pretty confident that you are in an area that was once glaciated.