Fealty in the SCA
Fealty in the SCA is an oath sworn by peers of the realm to the Crown and Kingdom, represented by the person of the King and Queen. Often, Fealty may be sworn to other powers that be, such as a Prince or Princess, or an apprentice, squire, or protege might swear fealty to their peer.
Generally, Landed Barons and Baronesses must swear fealty since they hold lands from the Crown.
Fealty is generally indicated by the wearing of a plain chain of links either as a necklace, or a small linked chain under their peerage badge. It is thus considered slightly out-of-place to wear such a plain chain if you are not a peer and have not sworn fealty. Note that the customs surrounding the wearing of chains of fealty is a subject to InterKingdom Anthropology.
It is also worth noting that anyone may swear fealty to the Crown (or to anyone for that matter), but public oaths are often limited to knights or peers simply to keep Court to a reasonable length.
Kingdom of Lochac
In the Kingdom of Lochac, Kingdom Greater Officers are required to swear fealty, whereas Kingdom Lesser Officers are able to swear fealty should they choose. The laws of the Kingdom of Lochac prohibit fealty being required to any body lower than a Principality, i.e. Barons and Baronesses may not require that their subjects swear fealty.
The Middle Kingdom
In the Kingdom of Middle, landed Barons and Princes must swear fealty every reign in person to either the King or his appointed representative. Greater Officers, and Knights may choose to swear fealty or not.
Copies of the oaths: See the kingdom's Laws at http://www.midrealm.org/seneschallorum/midrealmlaw.doc