What is a natural lake? Salt water? Fresh water? Both?

Most lakes are fresh water, but there are a few exceptions. (i.e. The Great Salt Lake in Utah)

A natural lake could be fresh or salt water as long as it wasn't man made. For an example, the Caspian Sea, considered a lake by a few, is completely natural and is salt water. There's also Lake Baikal which is salt water. And many more lakes that are fresh water.

A natural lake is any lake that occurs naturally.

Most lakes are fresh water. Usually, they were carved out of the ground by glaciers, then filled with glacial melt water and rain. Some were carved out by glaciers or geologic processes, and then filled with spring water form the ground.

Salt water lakes, or inland seas, formed when ancient oceans receded, and left water behind in low areas. Small salt lakes can also occur when ground water flows through salt deposits in the ground, and then comes to the surface as a spring which feeds a lake.