Longshore currents, or longshore drift as it is more commonly referred, is the zig-zag movement of sediments (sand and other materials) along a coast parallel to the shore.
Waves carry sediments up onto a beach in the swash and down again in the backwash. When waves approach the shore at an angle, the swash of the waves up onto the beach deposits the sediments along the beach in this same direction. The backwash of the waves (under the influence of gravity), moves and deposits the sediments straight back down off the beach, at right angles to the shore. This produces the zig-zag movement of sediments along the beach. The larger, heavier particles are found where the process begins and the smaller, ligher materials are more easily carried with the waves.
Where waves are strong, sediments are carried away and the coast will be eroded; where waves are weak, sediments will be deposited. The continual actions of waves therefore changes the shape of the beach and the coastline. Longshore drift also carries marine debris and other materials along the coast.