So far as I know, there's only one woman who we can say with any reliability bore arms in battle and who was probably trained in their use. This is "Big/Great Margot", who was the Flemish standard bearer at the Battle of Westrozebeke and who died during the battle. That she was armed and probably trained in the use of weapons can be inferred from the fact that she acted as the standard bearer; this was a very prestigious position and was given to those with the best skill of arms, as having the standard fall usually caused the army to break. Placing someone who could not fight well in that role, therefore, would have been potentially disastrous to the army. With that said, this is all just guesswork and Great Margot need not necessarily have been skilled with arms.

Far more frequently, women acted as commanders, either in their husband's name, in the name of their son or in their right as a widow. Jean A Truax has made the case that Anglo-Norman women were expected to lead soldiers in the absence of their husbands, for what ever reason, and that it was thought they were perfectly capable of making broader strategic decisions. In some cases, they would dressed in mail and carried symbols of their military authority, such as Sibyl, the wife of Robert Bordet who

kept sleepless watch; every night she put on a hauberk like a soldier and, carrying a rod in her hand, mounted on to the battlements, patrolled the circuit of the walls, kept the guards on the alert, and encouraged everyone with good counsel to be on the alert for the enemy’s stratagems.

Isabel of Conches/Tosny is probably a part of this group, but Oderic is ambiguous enough that it's possible she did fight. Sikelgaita, on the other hand, is almost certainly in the same group as Sibyl. While she is portrayed wearing armour and charging fleeing Normans with a lance to rally them during the Battle of Dyrrhachium by Anna Komnene, and is mentioned in William of Appulia as participating in other campaigns with her own men, Sikelgaita's use of the lance was more likely to have been to issue commands than to charge her enemies.

More often, the women who lead sections of armies did not wear armour and probably weren't present on the field of battle itself, but were close by it. Matilda of Tuscany, along with her mother Beatrice of Lorraine, Adelaide of Susa, Matilda I Queen of England, the Empress Matilda, Matilda II Queen of England, Aethelflaeda, Jeanne de Penthievre and probably many others I'm not aware of, fall into this category. Though they may have ruled their domains with an iron fist, dictated strategy, ordered specific attacks and how they should be carried out, etc, they were not present for the actual carrying out of their orders and delegated this to trusted commanders.

There were some circumstances where women fought in combat. Mostly, this occurred during sieges and generally on the defender's side. The women of the Catalan Company, for example, were armed with spared armour and crossbows during the Siege of Gallipoli and apparently acquited themselves quite well and impressed Ramon Muntaner, their commander and later the chronicler of the Campany's adventures. At Toulouse in 1218, Simon de Montfort (whose wife Alix was also one of those women who commanded troops on campaign) was killed by a siege engine operated by the women of the town, while at Drincourt in 1167 women are recorded as coming out and fighting along side the men to drive the French from the town. Jeanne de Montfort actively encouraged the women of Hennebont to climb to the battlements and throw stones down onto the besieging French when they attacked, while the women of Beauvais defended the town from the Burgundians in 1472 by throwing burning brands down on them and Jeanne Hachette lead a counter charge against some attackers who had gained the wall. Interestingly, in the French Midi during the Hundred Years War, women who were citizens of a town were expected to serve as part of the militia if they had no husband or son of the appropriate age and could not find a hired replacement. Though this doesn't mean that these women actually trained in arms, it might have provided them (or any adult daughters) with an opportunity to do so if they had the desire.

Even when not actively fighting, though, women still provided vital support. Apart from praying - which was critical to warfare in the medival view - they risked life and limb by carrying supplies to the front lines, nursed the injured, provided labour for the construction or destruction of defences, foraged for food or materials, served as washer women and barbers, bought and sold food to the army, occasionally helped keep watch (along with other non-combatant men) if numbers were low, and, yes, the inevitable role of prostitution. This could be ad hoc, just following the army and plying any number of other trades including prostitution, or it could be more in line with late 15th century practice, where the prostitutes were mounted and officially belonged to a certain section of the army and who also acted as maids.

So, to sum up, some women probably did fight on the front lines, but these would have been very rare. More common, but still rare, were those who commanded from behind the front lines dressed in armour and carrying military regalia. Far more common were those who did not wear armour and mostly commanded from beyond the battlefield - but who were still close by. Finally, there are those who didn't train to any great degree, but who would fight from the tops of walls or in a mob during attacks in towns or cities. In addition to these women who fought were the most numerous women: those who supported the warriors, either through providing supplies on the battlefield, washing their clothes, providing labour, selling them food or by providing physical or emotional release via prostitution.

References

"Anglo-Norman Women at War: Valiant Soldiers, Prudent Strategists or Charismatic Leaders?", The Circle of War in the Middle Ages by Jean A Truax

"Women in Medieval Armies", Journal of Medieval Military History, Volume 4, J.F. Verbruggem

The Art of Warfare in Western Europe During the Middle Ages, by J.F. Verbruggen

The History of William Marshal, tr. by Nigel Bryant

‘An Entirely Masculine Activity’? Women and War in the High and Late Middle Ages Reconsidered, by James Michael Illston

The campaigns of Countess Matilda of Canossa (1046-1115), an analysis of the history and social significance of a woman's military leadership, by David Hay

Town Defence in the French Midi during the Hundred Years War, by Noel, R.P.R.

The problem is that most nobles at the time would not hire peasants due to their poor fighting skills and disorganisation and noble women would not be allowed to fight or train for fear of breaching social standards of the time. The historian Lutkin would argue that, at least in England in the 1450-87 period some women fought, but she is certainly an exception. The main issue is that women fighters would not be recorded as nobody would record the fighters; if you look at the War of the Roses, there is virtually no documentation mentioning anything other than baronial feuds. In summation, these cases are extremely rare (with notable exception of Joanne of Arc in Orleans in 1428).

However, though women may not have fought, they often lay at the rear of armies pulling supply wagons, tending to the wounded and provided prostitution. Their support role, though understated, was invaluable.