From Roger of Hovedon on the Battle of Gisors:
After these events, Robert, earl of Leicester, came with forty knights and a few men-at-arms, before the castle of Pasci, which had belonged to him; on which the knights of the castle sallied forth with a great number of men-at-arms and the citizens of the town, for the purpose of capturing him and his followers; and he, being able to offer them no resistance, escaped with difficulty, and lost four of his knights.
*Note: The Annals of Roger of Hoveden provide an account of the battle of Gisors between Richard I of England and Philip Augustus of France, which took place in 1198. Hoveden writes down two versions of this battle, and includes a letter written by Richard I to the bishop of Durham, that details the battle.
A man-at-arms was a professional soldier, well-trained and equipped for war, and while all knights were in prinicpal men-at-arms, not all men-at-arms were considered knights. There was a distinction, as is clear in the above passage from Roger of Hovedon: “Robert, earl of Leicester came with forty knights and a few men-at-arms.”
Men-at-arms were, for the most part, below the class of knights in terms of wealth and status and weapons. In England, men-at-arms included sergeants, esquires, and valets. In France, they were most often referred to as sergeants only. A man-at-arms did not have the retainers that a king or higher lord might have, and they could not afford the same quality armor or horses. Men-at-arms mostly rode rounceys or coursers, instead of the traditional destrier of the higher-born nobles. Such animals were far too expensive. Sometimes, a man-at-arms might get lucky and take the horse of a captured or dead knight.
Oftentimes, men-at-arms would ride to battle and then dismount to fight, not so different than the style used by the Anglo-Saxon armies of King Harold Godwinson’s days. The English men-at-arms fought on foot at the Battle of Crecy according to the Chronicles of Jean Froissart:
The Englishmen, who were in three battles lying on the ground to rest them, as soon as they saw the Frenchmen approach, they rose upon their feet fair and easily without any haste and arranged their battles. The first, which was the prince’s battle, the archers there stood in manner of a herse and the men of arms in the bottom of the battle. The earl of Northampton and the earl of Arundel with the second battle were on a wing in good order, ready to comfort the prince’s battle, if need were.
Even though they did not ride the finer warhorses, men-at-arms were by no means considered light cavalry. They made up a good portion of an armies’ medium to heavy cavalry, and by the beginning of the 13th century, the dinstinction between men-at-arms and full knights became less obvious, as many minor nobles chose to forgo the dubbing ceremony of knighthood in order to avoid the obligation of 40 days free military service. This narrowing of the classes marked an important shift from the traditional feudal military society to a system of paid military recruits or mercenaries.
Sources:
Nolan, Cathal J. The Age of Wars of Religion, 1000-1650. Greenwood Publishing Group, 2006.
The Annals of Roger Hovedon, on the Battle of Gisors (primary source)
The Chronicles of Jean Froissart, An Account of the Battle of Crecy (primary source)