The Great Lakes are the world's largest group of freshwater bodies, hosting a large number of species within their waters and on their shores, but also sustaining life over large areas of North-America thanks to their tributaries' impact on land ecosystems.
Because they're part of a very large land-mass, their size is not always evident. Sometimes the best way to get an idea of the scale of something is to change the context. That's why we like this map from Great Lakes Guru that shows the Great Lakes super-imposed over Europe. It gives a better idea of just how frigging big they are!
For more details, the EPA has a good page about the Great Lakes.