The Great Lakes are a group of five very large bodies of water located on the border between the United States and Canada, except for Lake Michigan, which lies entirely in the USA. They aren't called 'Great' for nothing. Lake Superior is the largest body of fresh water in the world. These are truly big lakes. They contain something like 25% of all the fresh water in the world, which is a lot of water.
The Great Lakes are connected to the Atlantic Ocean by the St Lawrence River to form a continuous seaway stretching from the coast to the heart of the continent. This seaway is great for commerce, allowing ships from around the world access to such cities as Cleveland, Detroit, and Chicago in the USA and Toronto, Hamilton, and Montreal in Canada.
The Great Lakes are a great tourist attraction, allowing swimming, beaches, and other seaside activities without some of the oceans' inconveniences, like salt water and sharks. Relatively small lakeside towns, such as South Haven, Michigan, USA and Port Dover, Ontario, Canada experience huge crowds in the summer tourist season, and hundreds of people on the beaches every day (or at least every nice day). The Great Lakes' commercial fishery, once a major industry, has been reduced to a fraction of its former size by such factors as pollution, parasites (such as the Sea Lamprey), and competition from introduced sportfish stock.
Another interesting feature of the Great Lakes is the large number of lighthouses surrounding them. Sadly, these old manned lighthouses, originally created for the purpose of helping navigation (as in, making sure boats don't just plough right onto dry land) are now largely obsolete1. However, they are still a great tourist attraction, with hundreds, if not thousands, of people coming to look at them every year. They are often viewed as important historical sites.
There are many advantages to having the Great Lakes around, including commerce, recreation, and tourism. However, these natural wonders can get a little cranky sometimes. There have been an astonishing number of shipwrecks on the Great Lakes. While not as many ships sink there now as in the colonial days, small boats will occasionally be lost; and even large ships, such as the famous Edmund Fitzgerald don't stand much of a chance in the November storms on Lake Superior2.