Many people would like to experience something of a favorite historical period. Perhaps they would like to envision themselves as a knight on crusade, a backwoodsman scouting through the forest, a wily renaissance politician, a bold Viking, or even a skilled craftsman creating objects of classic beauty with simple tools and great skill.
There are a number of ways for people to do this. Each of them has good and bad points, and they are not necessarily mutually exclusive. They are all generally great fun however.
A reenactment is a show put on by volunteers (usually) which recreates some event in history. Participants are required to maintain a defined level of accuracy during the show. That level varies with the different groups, but is generally very high. When the public and cameras leave however, the standards relax.
Living history is an attempt to relive part or all of an historical period by recreating the conditions of the time. It is done for the participants, not observers. Again, a defined level of accuracy is used. It may include the total environment, as in "buckskinning", where the camp, clothing, equipment, etc. must be within acceptable standards. Or it may be limited to some aspect of historical life, as when a craftsman in jeans and a T-shirt knaps a flint dagger, forges damascus steel, letters a parchment, or brews mead. It can extend to a period treck in the wilderness using only the equipment and skills of our ancesters, or the recreation of a working Viking longship. There are many living history groups listed in the bookmark file.
And then there is the SCA. The SCA is by far the largest group of people interested in directly experiencing history. However it is neither of the above. It falls somewhere between living history and an historical theme party. When you join the SCA you choose a "persona", a fictional person who could have lived in the period of your interest. You "become" that person. The required standards of dress are minimal, but you are expected to behave as your persona might. When you earn recognition or awards in the SCA, you do so for real deeds. The SCA is neither living history, nor reenactment. Perhaps it could best be described as "historical play". Some people take some of their play very seriously however, and some really excellent research and recreation is done in the SCA.
In the end the SCA is easily the largest early history group. It's available everywhere, and very easy to join. If you have an interest in early history, don't miss it! You can always join a more specialised group later.