History of the Anchor.
The earliest anchors were probably rocks and many rock anchors have been found dating from at least the Bronze Age. Many modern moorings still rely on a large rock as the primary element of their design. However, using pure mass to resist the forces of a storm only works well as a permanent mooring; trying to move a large enough rock to another bay is nearly impossible.
A fisherman’s anchor
CQR (Clyde Quick Release)/Plough
Designs of temporary anchors
A modern temporary anchor usually consists of a central bar called the shank, and an armature with some form of flat surface (fluke or palm) to grip the bottom and a point to assist penetration of the bottom; the position at which the armature is attached to the shank is called the crown, and the shank is usually fitted with a ring or shackle to attach it to the cable. There are many variations and additions to these basic elements—for example, the whole class of anchors which include a stock such as the fisherman and fluke anchors.
The range of designs is wide, but there are actually trends in designs for modern anchors which allow them to be classed as hook, plough, and fluke types, depending on the method by which they set.
Hook designs use a relatively small fluke surface on a heavy, narrow arm to penetrate deeply into problematic bottoms such as rocky, heavy kelp or eel grass, coral, or hard sand. Two of the more common versions of this design are the fisherman and the grapnel.
Plough designs are reminiscent of the antique farm plough, and are designed to bury themselves in the bottom as force is applied to them, and are considered good in most bottom conditions from soft mud to rock. North sea designs are actually a variation of a plough in how they work; they bury into the bottom using their shape.
Fluke designs use large fluke surfaces to develop very large resistance to loads once they dig into the seabed. Although they have less ability to penetrate and are designed to reset rather than turn, their light weight makes them very popular.
In the past 20 years or so, many new anchor designs have appeared. Driven by the popularity of private pleasure boats, these anchors are usually designed for small to medium sized vessels, and are usually not appropriate for large ships. See modern designs.
A fisherman’s anchor
A traditional design, the Admiralty Pattern, "A.P.", or simply "Admiralty", and also known as "Fisherman", is the most familiar among non-sailors. The design is a non-burying type, with one arm penetrating the seabed and the other standing proud. The anchor is ancient in design and has not changed substantially over time.
It has a good reputation for use in rock, kelp and grass. On soft grounds it is important that the anchor has large flukes. Even with large flukes, it has less holding power to weight ratio than modern anchors. It is a good all around anchor for those who need to anchor on all types of seabed. However, if anchored under conditions where the boat can drift over and around the anchor (light, changing wind and tides), the anchor is easily fouled by the chain and the anchor loses its holding power. Under these conditions, the anchor needs to be brought to surface and checked before it can be trusted in any but light conditions.
The A.P. is more difficult to stow than modern anchors, as it does not stow in a hawse pipe or over an anchor roller. Most versions include a folding stock so the anchor may be stowed flat on deck. Some are also made with hinged flukes and some in 3 pieces and can thus be stowed in a locker.
A fluke-style anchor
The most common commercial brand is the Danforth, which is sometimes used as a generic name for the class. The fluke style uses a stock at the crown to which two large flat surfaces are attached. The stock is hinged so the flukes can orient toward the bottom (and on some designs may be adjusted for an optimal angle depending on the bottom type). The design is a burying variety, and once well set can develop high resistance. Its light weight and compact flat design make it easy to retrieve and relatively easy to store; some anchor rollers and hawse pipes can accommodate a fluke-style anchor.
The fluke anchor has difficulty penetrating kelp and weed-covered bottoms, as well as rocky and particularly hard sand or clay bottoms. If there is much current or the vessel is moving while dropping the anchor it may "kite" or "skate" over the bottom due to the large fluke area acting as a sail or wing. Once set, the anchor tends to break out and reset when the direction of force changes dramatically, such as with the changing tide, and on some occasions it might not reset but instead drag.  Additionally, the articulating flukes may be vulnerable to jamming; in testing the Fortress FX-23, Voile magazine (France) wrote that a small stone was sufficient to jam the fluke.
A traditional design, the grapnel style is simple to design and build. It has a benefit in that no matter how it reaches the bottom one or more tines will be aimed to set. The design is a non-burying variety, with one or more tines digging in and the remainder above the seabed. In coral it is often able to set quickly by hooking into the structure, but may be more difficult to retrieve. A grapnel is often quite light, and may have additional uses as a tool to recover gear lost overboard; its weight also makes it relatively easy to bring aboard.
Grapnels rarely have enough fluke area to develop much hold in sand, clay, or mud. It is not unknown for the anchor to foul on its own rode, or to foul the tines with refuse from the bottom, preventing it from digging in. On the other hand, it is quite possible for this anchor to find such a good hook that, without a trip line, it is impossible to retrieve. The shape is generally not very compact, and is difficult to stow, although there are a few collapsing designs available.
CQR (Clyde Quick Release)/Plough
So named due to its resemblance to a traditional agricultural plough (or more specifically two ploughshares), many manufacturers produce a plough-style design, all based on or direct copies of the original CQR (Secure), a 1933 design by mathematician Geoffrey Ingram Taylor. Owing to a now well established history, ploughs are particularly popular with cruising sailors and other private boaters. They are generally good in all bottoms, but not exceptional in any. The CQR design has a hinged shank, allowing the anchor to turn with direction changes rather than breaking out, and also arranged to force the point of the plough into the bottom if the anchor lands on its side. Another more recent commercial design, the Delta uses an unhinged shank and a plough with specific angles to develop slightly superior performance. Both can be stored in most regular anchor roller systems.
Owing to the use of lead or other dedicated tip-weight, the plough is heavier than average for the amount of resistance developed, and may take a slightly longer pull to set thoroughly. It cannot be stored in a hawse pipe.
The genuine CQR and Delta brands are now owned by Lewmar, although they have both been on-sold several times during their lifetimes.