According to National Geographic, a jetty is a long, narrow structure that protects a coastline from the currents and tides. It is usually made of wood, earth, stone, or concrete. They stretch from the shore into the water.
Jetties protect the shoreline of a body of water by acting as a barrier against erosion from currents, tides, and waves. It can also be used to connect the land with deep water farther away from shore for the purposes of docking ships and unloading cargo. This type of jetty is called a pier.
Jetties can be popular tourist attractions. They usually provide safe access to coastal areas. The most famous jetty is probably Spiral Jetty, a large sculpture created by the artist Robert Smithson in 1970. It is located on the northeast shore of the Great Salt Lake, in the U.S. state of Utah.
Check out The World's Most Dramatic Coastlines, featured at Huffington Post.