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.
In the SCA, all Knights are required to swear fealty to the Crown. Masters at Arms are also peers of the Order of the Chilvary, however reserve the right to swear fealty instead. The other peers (Laurels and Pelicans) are given the right to swear fealty if they choose, and will be called the same thing regardless of their choice.
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.
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.
The oaths of fealty (as the Middle has many variances) can be found in the Laws of the Middle Kingdom, linked at http://www.midrealm.org/seneschallorum/midrealmlaw.doc
Other random notes on fealty: