Barrier islands are long, thin, and sandy islands that run parallel to the coast. Between a barrier island and the main-land, there will be a calm, protected body of water such as a lagoon or bay.
In order for barrier islands to form, several conditions must be met. First, there must be a source of sand to build the island. Next, the coastline must have a broad, gentle slope. Finally, the forces of waves, tides, and currents must be strong enough to move the sand, so as to form an island.
Once formed, barrier islands are dynamic. The winds and waves constantly rework, and move the barrier island sand. Changes in sea level also affect these islands. Each barrier island has a shoreline that faces the open sea, and receives the full force of waves. This shoreline is often called the beach. Behind the beach, there will be sand dunes.