The icefield is a huge area of glaciers that are connected together. So, the icefield is larger than a glacier and basically, several glaciers are connected together to form an icefield.
These icefields come into being as a result of a formation of snow, which has been compressed through the years. Although there are several glaciers that come together to form an Icefield, each of these glaciers are distinct from each other. It is only the fact that they are interconnected with other glaciers to form an icefield.