The thermocline is a layer within a body of water where the temperature changes rapidly with depth.
Because water is not perfectly transparent, almost all sunlight is absorbed in the surface layer, which heats up. Wind and waves circulate the water in the surface layer, distributing heat within it somewhat, and the temperature may be quite uniform for the first few hundred feet.
Below this, however, the temperature drops very rapidly-- perhaps 20 degrees C with an additional 500 feet of depth. This area of rapid transition is the thermocline.
Below the thermocline, the temperature continues to drop with depth, but far more gradually. In the Earth's oceans, 90% of the water is below the thermocline. This deep ocean consists of layers of equal density, being poorly mixed, and may be as cold as 0 to 3 degrees C. 
The thermocline has been important in submarine warfare, because it can reflect active sonar.
When scuba diving, a thermocline of a few degrees can often be seen between two bodies of water, for example a colder upwelling or current running into a surface layer of warmer water. It gives the water an appearance of the wrinkled glass that is often used to obscure bathroom windows.
1. "Temperature of Ocean Water", University Corporation for Atmospheric Research, University of Michigan. (http://www.windows.ucar.edu/tour/link=/earth/Water/temp.html)