A pond is the body of water that is smaller than a lake. To be classified as a pond it has to be standing water, water that is not running, and this can either be naturally occurring or man made.
They are usually very shallow, and many people chose to have them in their garden as a decorative element, allowing them to have a range of aquatic plants and wildlife.
When occurring naturally, a pond can form as either part of a water system, so in marshy land that a river runs through, or completely alone in just a dip in the ground that fills up with water. Alligators and beavers can also sometimes create ponds by digging in a wet area.
A man made pond can be created for many different reasons, it doesn't just have to be decorative. People also use them to store energy, breed fish and observe and protect wildlife.
One of the technical definitions of a pond is 'bodies of water where light penetrates to the bottom of the waterbody'. So this is reference to how shallow the water is, and not the size of it. This definition, along with others, has been met with disapproval, as this is not the only characteristic of a pond.
There is actually no world recognised size that makes a pond a pond. So the size that a pond needs to be to turn into a lake is unknown.