The Society for Creative Anachronism (or SCA for short) is a hobby organization devoted to studying and re-creating the Middle Ages and Renaissance, primarily in Western Europe but also other regions such as the Middle East and Japan. It could be considered a historical reenactment or living history organization, but, unlike most such organizations, the SCA does not concentrate on a narrowly-defined time period, let alone a specific war or other event. The SCA also does not have a minimum standard of authenticity. The activities of the SCA have more to do with "re-creation" (and recreation) than with re-enactment. Some SCA participants describe the SCA as a large group of people with interlocking hobbies.
At a typical SCA event, you will see a wide range in the quality of re-creations and reenactments. Some individuals may do some aspects of re-creation well (say, music), while simultaneously doing others poorly (say, clothing). Still, there is one standard that all participants of official SCA events are held to: participants should try to dress in pre 17th century garments. In SCA jargon, this era is called "Period", although some insist that "Period" only legitimately refers to the millennium of CE 600-1600.
The relatively low requirements to participate are also one of the strengths of the SCA. As long as you are genuinely interested in medieval or Renaissance life, there is a place for you in the SCA. If it was a job or a hobby in Period, most likely someone within the SCA practices it, sometimes to an excellent degree. Excellent practitioners are awarded a SCA "peerage" (SCA-wide, highest level award) known as the Order of the Laurel. Other "peerages" are the \Order of the Pelican (awarded for service to the organization) and the Master of Arms/Knight (awarded for excellence in martial arts). This use of "peerage" is unique to SCA and should not be misconstrued to have any resemblance, whatsoever to historical "peerage" as once practiced in the United Kingdom or similar countries, though they are inspired by medieval concepts.
The SCA is worldwide in scope. There are active groups all over the USA, Canada, Europe, Australia, and New Zealand with scattered groups elsewhere. The Known World is divided into Kingdoms. Each Kingdom determines its King and Queen by combat in a crown tournament.
To prepare for tournament and wars, some groups hold "fighter practices" where a full contact martial sport is practiced. The fighters wear armor (often of their own making--very often of plastic) and strike each other with weapons primarily made of rattan and made to resemble swords, pikess, spears, axes and other medieval weapons. This 'Heavy Weapons Fighting' is interesting among martial arts for its lack of formalized training. This allows new-comers to learn at their own pace and to add to the game in unpredictable ways. Typically several years of direct experience in the SCA's Heavy Weapons Fighting are needed to excel in tournaments. SCA Heavy Weapons Fighters also practice many-on-many engagements called melees or wars.
Some groups practice a style of fencing with rules unique from Olympic fencing rules. SCA fencing does not reconize lanes or right-of-way. It is an attempt at "re-creating" rapier combat of the 16th and early 17th centuries. Some SCA practitioners of this art have become recognized worldwide as scholars in the field outside of the SCA. However, overall knowledge is spotty, at best.
Some SCA groups hold regular classes in various medieval arts and sciences are practiced. These might include dance practices where Renaissance Dance and English Country Dance is taught, or calligraphy and illumination workshops where interested people study and create illuminated manuscripts. At large gatherings, such as the annual Pennsic War, many people offer classes on various aspects of medieval life.