River, Lake,Lagoons and Pond are body of water and the main difference is water movement. Commonly, rivers flow in only one direction towards ocean and rest of the waterbodies are standing and available in significant sizes.
Rivers are flowing water bodies that flows into the ground towards ocean,sea or towards another river. Rivers are part of the hydrological cycle and main source of water for people and animals.
Lake is a still water body surrounded by land and most found in mountainous areas of India, feed by rain water or streams. Lakes are fresh water,Salt lakes,,Crater lakes,Glacial lakes,natural lakes and can be artificial.
Lagoons are shallow water bodies, get separated from a larger water body by an islands, barrier or reefs. They can be man made and commonly called as coastal lagoons, Lagoons at the river or ocean mouth are classified as estuaries.
Pond is a standing body of water, usually smaller than a lake and home for aquatic plants and animals such as Mugger, pond heron and aquatic birds. Ponds are frequently human constructed and contain shallow water for aquatic plants and animals.