A lake really is just another component of Earth's surface water. A lake is where surface-water runoff (and maybe some ground-water seepage) have accumulated in a low spot, relative to the surrounding countryside. It's not that the water that forms lakes get trapped, but that the water entering a lake comes in faster than it can escape, either via outflow in a river, seepage into the ground, or by evaporation.

A reservoir is the same thing as a lake in many peoples' minds. But, in fact, a reservoir is a manmade lake that is created when a dam is built on a river. River water backs up behind the dam creating a reservoir.

Here's a question for you: when a beaver dams a creek, is the pond that it creates a lake or a reservoir?

The Earth has a tremendous variety of freshwater lakes, from fishing ponds to Lake Superior (the world's largest), to many reservoirs. Most lakes contain fresh water, but some, especially those where water cannot escape via a river, can be salty. In fact, some lakes, such as the Great Salt Lake, are saltier than the oceans. Most lakes support a lot of aquatic life, but the Dead Sea isn't called "Dead" for nothing -- it is too salty for aquatic life! Lakes formed by the erosive force of ancient glaciers, such as the Great Lakes, can be thousands of feet deep. Some very large lakes may be only a few dozen feet deep -- Lake Pontchartrain in Louisiana has a maximum depth of only about 15 feet.

Some of the salty lakes were formed in ancient times when they were connected to seas and when rainfall may have been heavier. These lakes have been shrinking since the last ice age. The ancient Lake Bonneville in the United States was once as big as Lake Michigan, and the Great Salt Lake was once about 14 times as large as it is now.

Information provided by: http://wwwga.usgs.gov