Since warfare is interwoven throughout mankind's history, period costuming and historical reenactment inevitably include the implements of war. These include both arms and armor. Just as the tools for killing people (arms) have changed over the centuries, becoming more effective, so has the protective equipment needed to keep from getting killed (armor). While this has been a gradual evolution, in general any given historical period can be identified by the arms and armor of its combatants. For example, you would not expect to see a knight in cumbersome metal plate armor in an American Civil War reenactment.
Over the centuries, various styles and materials have been used for the creation of personal body armor and shields. These include leather, wood, and even silk. The best known and most obvious material for this use is metal. According to Colonial National Park Service, some of the earliest armor, or armour, was a type known as lamellar. This consisted of metal plates (later metal rings) sewn to a garment. Armor later evolved in two different directions. The small metal plates became large metal plates formed to fit over specific portions of the body, breastplates and backplates for example. This is the type of armor most often called to mind when we think of a knight in shining armor. However, this kind of armor was stiff and cumbersome. A more flexible type of armor was created by using just the metal rings, without sewing them onto a cloth garment. This is what is known today as chainmail (sometimes known as chain mail, chainmaille, or just maille).
Although chainmail is believed to be a Celtic invention, the Romans adopted it in about the 3rd century B.C.E. A thousand years later, it was being used throughout Europe. At the same time, a different style of maille was being developed in the Orient, specifically Japan.
The principle behind chainmail is the linking of metal rings. Connecting rings together in this way is sometimes referred to as knitting or weaving maille. Although this calls to mind the way a chain is made, the metal rings are linked in much more elaborate ways than a simple chain. You might compare this to the difference between tying a knot and knitting a sweater. The rings are connected together to form sheets. In turn, these sheets are used to form garments. Over the centuries and throughout the world, many different patterns have been formed. Some of these are for practical purposes, such as to add strength to the maille. Others may be thought of more as stylistic or decorative elements.
Making chainmail today is probably one of the easiest and most accessible ways to become involved in making historic armor. The tools, materials, and methods used today are practically identical to those used in the middle ages. The exception is that many armorers today use modern metals for their advantages in corrosion resistance and weight. These include aluminum, galvanized steel, and stainless steel.
For those interested in the creation of historically accurate arms and armor, the Internet is a good resource. For example, Dylon Whyte's Art of Chainmail gives some historical information on armor in general, and some nice illustrations on the creation of chainmail. Other helpful sites related to the creation of chainmail are Maille Artisans, Chainmaille Fashions, and Butted Mail: A Mailmaker's Guide.
Eventually the craftsmen creating chain maille realized that the same methods and patterns could be used to create jewelry. In recent years, chainmail jewelry has had something of a renaissance.
So then, whether you are a historical reenactor or are just interested in unique jewelry, the ancient art of chainmail might be something that you would like to try.