Households, within the context of the SCA, can be defined as non-geographical groups of people who have joined together to share in their SCA experience. Households can be as small as two people, or have hundreds of members from many kingdoms.
Many people do not belong to households, nor is household membership necessary in order to have a rewarding SCA experience, but many people enjoy the atmosphere households produce and the opportunity to develop a second "family".
Structure differs from one household to the next; it is up to the leaders or founders of the household to decide on the political structure. Some households just consist of friends who get together for a good time, others organize themselves to form a genealogy, with brothers and sisters and great aunts, and many are persona specific. The one thing that most households have in common is that they are joined by invitation only.
Usually households require a certain amount of loyalty, and if you are a newcomer, you may not be aware of the alliances that come with joining a household. Consider carefully what will be expected of you as a member of any household, and don't commit yourself rashly.