From the Rulebook
At-Arms and Pages: At-Arms or Pages are usually individuals who are sworn to Knights, Squires, or Nobles. In Amtgard, being an At-Arms or Page denotes a special relationship between the individual and their mentor,who is typically a Knight, Noble, or Squire. At-Arms can generally be recognized by a black belt with silver trim or a green belt. Pages can generally be recognized by a yellow belt. At-Arms may also go by Man-at-Arms, Woman-at-Arms, Comrade-at-Arms, Sword-at-Arms, Standardbearer, Shieldmaiden, ShieldBrother, Cleric or other similar terms.
Historically, a man-at-arms was a fighter employed by an army, lord, or individual knights. In Amtgard, men-at-arms are roughly analogous to pages, in that they are taken by squires. Although this is theoretically a mentor-student type relationship, there are a multitude of reasons that a squire might take a man-at-arms.
Men and Women at Arms wear a black belt. In some Kingdoms this is further established with silver trim but many places don't require or use it.
Everything about At-arms is custom, so there are no hard and fast rules.
The difference between a Man at arms and a page:
Officially, there is no difference, as neither is officially recognized in the Order of Precedence or most Corporas. Some kingdoms define a man-at-arms as some one who fights, while a page is a non-combatant. Some kingdoms eschew one in favor of the other. Some kingdoms allow squires to take a man-at-arms, and a noble to take a page. In some places, children too young to fight are pages, while everyone else is a man-at-arms.