I'm ignoring the high medieval era as warfare becomes to complex for the concise all encompassing statements I want to make so I don't end up writing a paper on this.

A charge of cavalry either decimating a formation of peasant infantry or striking another mass of heavy cavalry. Imagine two hammers striking each other but one man is The Rock and the other is a pre pubescent boy. The weaker force (doesn't mean they lost; they simply didn't win the charge which could be due to a number of factors) would be driven back and progress would be made in taking "ground".

However, the battle is not over as that was just the initial charge The men at arms and knights if they were not already in a wedge formation would form miniature wedges of their own as naturally superior fighters proceeded to carve their way in one side or the other so your battle line wouldn't be a straight line it would be like a jagged zig zag line as both sides found advantages in various ways. Usually the commander, if he wasn't a moron would have two things going on. He would be at the rear (or in the thick of things trying to stay with his trumpeter or someone using a flag notification system; both have obvious merits) with a reserve force of men he hadn't committed to the force. He would be using these men to punish any advantage he seea in the enemies lines or if he was losing to shore up one of the flanks or the center. Also he would likely have made either flank (or sometimes the center) hold more men so they had a greater chance of turning the enemies flank which is a death sentence in war to this day.

Eventually one side would begin to break and it would turn into a rout where you would see your individual combats as some stayed and fought and were captured, some were cornered and wouldn't surrender, and some attempted to fight their way out and flee.

Throughout the battle however the men are slowly losing more and more cohesion and the lines become more muddled and yes they would often be working in close knots of household troops and kin.

HOPE THIS HELPED!

Consider this fact: from the fall of the Roman Empire onward until about the thirteenth century there was very little central, nation-style governance. Even those places that had once been Romanized lost a great deal of cohesion, for example Britannia and Gaul. Pre-Roman societies of Northern Europe, namely those of the Germanic and continental Celtic peoples, were written to have relatively warlike citizenry from whom some warrior skill could be drawn - they also had religious and social systems upon which status was often built through raiding, proving yourself in battle, or protecting a headman. Most of the time, however, these people were like, farmers, or tailors, or miners, or pretty much regular folks.

Advance past the fall of the Roman Empire and you still have some warrior societies but people are getting more and more agrarian. In most places, when peasant levies were raised by Lord Whosit for fighting Lord Whatsit over the treaty of Who-Knows-What, it is likely that these men have almost zero battle experience and almost zero useful equipment aside from like, a spear. Faced with a bunch of other men armed with spears pointing at you, there'd be a great deal of hesitation to attack. I'd say that when you have these battles, they probably are very, very hesitant to actually engage with each other in the melee.

There was a great post on reddit talking about...YES I FUCKING FOUND IT!!!

Here's the link - this guy gives a very good explanation.