An anchor is basically a rope, usually made of steel, used to attach a boat to the hull of a ship to prevent it from floating because of water or wind. In its original form, an anchor was simply a rope or chain used for fishing. The word derives from the Greek ancora, that means "to tie."
Today, however, the anchor can be used for more than just fishing. There are many applications in boating where it is important to have one. A good example would be to anchor a boat that is moored next to an oil rig. This makes it easy for a fisherman to fish from both the boats and the rigs.
In some situations, however, an anchor used for fishing may not necessarily be needed. Some boats are outfitted with a propeller that can actually move in a different direction. Even if the propeller doesn't move, however, it may be necessary to anchor a boat against the hull of the boat to keep it stable. This is especially true in the case of fishing boats. In addition, some types of boats, such as yachts, are actually designed to be out at sea. In these cases, an anchor will not be required to keep the boat afloat, but it will be needed to anchor a boat against the hull.