Context: My friend and I are having a discussion about military roles in medieval times compared to Fantasy RPGs like Dungeons and Dragons, or Pathfinder.
Edit: I suppose I should clarify now to note I'm primarily interested in the early Medieval period from 400AD to 1000 or so AD, but information about the later Medieval Period would of course be welcome.
It depends on where you're looking, and when (because there are some seriously large changes in military and political organization in the 600 years you're talking about) but in Scandinavia, generally, you would have things be broken down like such:
A posse comitatus belonging to the nobility or extremely wealthy. Essentially armed retainers who were professional or semi-professional warriors. They would often fulfil other roles in the court/household of their master, but were usually administrators rather than farmhands.
Independent professional warriors - normally sea-kings (who were basically professional víkingar,) duellists, outlaws, or berserkír - who would serve whomever paid them the most.
Independent semi-professional warriors and traders/farmers/fishermen, who would go víking and then return to their day-jobs, as it were, afterwards. This is normally where you find most of the saga heroes from Iceland - people like Egill Skalla-Grímsson, Gunnar Hámundarson, etc.
The leiðang or fyrð, depending on where you were, which were essentially conscripted farmers and fishermen who would serve at Royal Request for a season and then return home. This existed in the early part in Denmark only, then as power was consolidated in Norway and Sweden, they introduced similar systems. Iceland never had anything like that.
Check out the articles by Inge Skovgaard-Petersen, Claus Krag, and Thomas Lindkvist in Knute Helle's The Cambridge History of Scandinavia, vol. 1: Prehistory to 1500 for a really good look at the political organization of Scandinavian kingdoms and how the military operated both before and during their organization.
Medieval Europe is a massive time frame ranging from about 400 to 1453 and some would even put it to 1492 and onward. Frankly summarizing over a thousand years of military history would be an impossible task; I couldn't even imagine trying to summarize my own area of study which is 300 years and that's just w.r.t. two areas of Europe let alone trying to describe everything and everyone.
Regardless we have had numerous AMA's about the Medieval Era and obviously warfare questions reign supreme. I'd recommend you two go through these and perhaps also consider narrowing your question down. Even just an 'early' or 'late' specification would make a world of difference along with a general region of interest. I'd imagine, for instance, the answer for Castille, Francia, Poland, the Eastern Roman Empire, Seljuks, and Muscovites would be totally different.
The AMA's:
AMA: Medieval Arms, Armor, and Military Equipment; 535-1453 CE
AMA: Late Antiquity/Early Medieval era circa 400 - 1000 CE, aka "The Dark Ages"
It may not all answer all of your questions but it should help give you a greater understanding of the period; the first AMA should be best for most of your questions regardless though.
I am the friend in this discussion. To clarify we were debating whether early middle ages basically just had knights, infantry, and peasants or if there where more varied specialized military roles.