Harbors can be man-made or natural. Man-made harbors will have a sea wall that protects the boats from rough seas and wind. Natural harbors will have land on most sides of the body of water. They also need to be in a position where the entrance is not facing the prevailing wind. (in this case the usual direction of the wind)

Some people confuse a port for a harbor. A port is a man-made place where boats and ships load and unload. In a harbor boats anchor or dock for short or long periods of time. A harbor can also have a port.

Related Terms
reservoir - you can have a harbor in a freshwater reservoir
coast - a harbor is located near a coast
island - many islands have a harbor
fjord - there is often a harbor within a fjord
port - there is often a port within a harbor
bay - a bay can be a harbor if the winds are not a problem
lake - large lakes sometimes have a harbor (usually man-made)