Modern metal detectors work beautifully with the standard coil and will find a quarter size target at 8 or more inches deep provided the machine is properly set up. Sometimes however, you may need a coil that will go deeper, or one that will be less affected by trash than the stock coils are. Generally speaking, larger coils will see deeper into the ground, and cover larger areas. Some drawbacks to larger coils are they can be less sensitive to coin size targets, and since they are larger and see more ground, they are also likely to see more trash, and therefore make it harder to determine good targets that are in the coils search field. They also tend to be a bit heavier.

Smaller coils allow you to search closer to metal objects like swings, slides, fences, and buldings than a larger coil will. Smaller coils are also better able to see between trash items. Therefore they may be a better choice in parks, school yards, or any place that has a high potential for trash. Drawbacks are usually less depth, and a smaller search field so it will take longer to cover an area.

Most stock coils are of a concentric (or circular) design and produce a balloon shaped search field, in other words, the signal will radiate out a bit from the side of the coil while the deepest signal will be below the center of the coil. There are elliptical (oval or retangular shaped) coils available that produce a wedge shaped search field, where the strongest signal is from the front (toe) of the coil extending to the back (heel) of the coil, and lose strength (and depth) near the sides.

Concentric coils usually go deeper than elliptical coils of comparative size, but they also have a smaller search field as you approach maximum usable depth than the elliptical coils. Detectorists that have used both designs usually prefer the elliptical design because of the shape of the search field, and the elliptical coils ability to separate trash from good targets. Elliptical coils generally have the ability to search nearer to metal objects. Elliptical coils are usually more expensive than concentric coils of the same general size. Both coil designs have thier place in the detecting world.

I'm NOT saying you need to rush out and buy after market coils, just be aware of what's available for your machine.