There are two commonly refered to lines along the beach. The first is the coast line and it is the line that appears on maps. It marks the foot of cliffs or dunes, and it is usually marked by a distinct change of slope. This is the most important area to measure and observe coastal erosion (often called coastal retreat). The second is the shore line, which is very hard to see and define. It is conventionally described as the conceptual intersection between the high watermark and the shore (beach) and is usually only loosely indicated on navigation charts or nautical maps. This line is really difficult to define in practice beacuse the high water line (MHWL, mean high waterline) is a fleeting event and as we will see changes over time.
The Beach (you may see it called shore in american texts) actually extends past the shoreline out to the low tide point (MLWL, Mean Low waterline). After that the sandy slope and water above it is know at the Littoral Zone. This extends out to the closure depth, the conceptual depth beyond which there is no longshore or cross shore transportation of sand. Usually the breaker zone extends out towards to this depth.