A nautical mile is a unit of measurement used in both air and marine navigation. It is equal to 1,852 meters, or about 6,076 feet. A nautical mile is slightly longer than a statute (land-measured) mile.
The difference between the two units of measurement originated with the need for a more precise way to measure distance at sea. In the early days of maritime travel, sailors used a device called a log line to measure speed and distance traveled. The log line was simply a rope with knots tied at regular intervals. By measuring how long it took the log line to play out behind the ship, sailors could estimate their speed.
However, there was no standard size for knots, so different captains would tie them at different intervals resulting in different estimates of speed and distance traveled. To solve this problem, the British Admiralty established that one knot should equal one nautical mile per hour (about 1.15 statute miles per hour). This meant that if a ship sailed at 10 knots, it would travel 10 nautical miles in one hour regardless of the captain's knot-tying skills!
Today we still use nautical miles to measure distances at sea because it makes calculation and navigation easier. Statute miles are used on land because they are more familiar to us; after all, we grew up learning how far things are in miles rather than nautical miles!