Answer:
If you have been to Venice (Italy), you have seen lagoons. In fact, the world lagoon is derived from a latin word laguna.
Principally, a lagoon is shallow water body which has been separated from the main water body due to sandbars, barrier islands, coral reefs,etc. However, sometimes, they includes other terms too such as bays, estuaries, sounds, or even lakes. They are of two types, i.e. Coastal Lagoons and Atoll Lagoons. In the figure attached, you can see how a lagoon looks like.