Pond loops are coils of geothermal loop pipe attached to a framework and sunk into the water of a pond (or lake). Pond loops are always closed loops. You cannot run surface water directly though your geothermal heat pump's heat exchanger because it will foul your heat exchanger (with algae, other bacteria, scaling, etc.).
Not every pond can be used for a pond loop. The pond/lake must be large enough in volume, and deep enough that the entire loop, when installed, will have 8 feet of water above it.
The pond can't be too far away from your home. If the length of pipe going to the pond would be long enough to satisfy the entire loop length requirement, then the pond isn't useful for your loop. Also, if the pond is far enough away to require fusion welding of the pipe, then DIYers shouldn't use the pond.
Pond loops cut down on loop costs considerably. A pond loop uses a lot less pipe than a horizontal earth loop to transfer the same amount of heat - and since they use less pipe, they also use less antifreeze. Pond loops also have much smaller excavation costs, because the only digging you do is to get the earth loop piping from your home to the pond and back again.