Actual chivalry is complex, bound up in the set of rules by which later period knights would not only do combat but also lead their everyday lives. All SCA fighters are required to behave in a chivalrous manner (see Rule 6 of Rules of the list). Everyone else should aspire to being chivalrous as well.
'Chivalry' is one of the ideals in which SCA aspires to taking the best of medieval life and recreate it. Essentially it is respect to others (and yourself), humbleness, graciousness and courtesy. A more precise definition is a little harder to come by, but many in the SCA have spent considerable effort at making a definition.
The Chivalry is a term used in the SCA to refer to one of the three great Orders of Peerage (the Order of Chivalry, the Order of the Laurel and the Order of the Pelican). The Order of Chivalry includes both Knights of the SCA and Masters at Arms. The primary difference between the two is that Knights swear fealty to the throne and wear a white belt and chain, whereas Masters at Arms swear no fealty and wear a white baldric.