Aside from the very definition of aristocracy in Western European cultures of the Middle Ages which has been debated a lot over the last century, stemming from the definition Marc Bloch gave back in the first half of the XX century, there was a clear difference between the nobility and the rest of the population, but it's very hard and near to impossible to provide a catch-all definition of all the potential iterations between these social groups. I will attempt to provide an overview of the matter.

Let's begin by saying that peasants did not have any legal status rendering them akin to slaves. Slavery as a overall concept is found quite rarely in the High to Late Middle Ages. I personally have found mention of African slaves in the Aragonese kingdom of Naples, but never were European peasants treated as such. Peasants who didn't own the land they worked and lived on (a sizable portion, most likely the majority) had certain duties towards the person who rented the land to them, in case said person was part of the landed aristocracy or was a member of the clergy or represented a monastery or church. In that sense, aside from the rent proper, in certain areas of Europe a peasant was expected to provide for example a given amount of workdays to work the lord's lands without pay (a very common practice in the Carolingian era for example). It was still the case in the late 1400s in southern Italy, where peasants working the land of one of the royal farmsteads were working for free according to an ancient custom tied to that land which had long fell out of fashion, to the point of convincing them to petition the royal fisc to obtain payment for their work (which they got, by the way, according to the notary who followed the case). The aforementioned obligations varied widely across the continent and appear to have changed over the course of time as the result of customary changes or negotiations between the communities and their local rulers.

With that being said, the aristocracy appear to have had a clear identity as a social group, a conviction of being different from the rest of the people and in that difference lay their inherent superiority over the peasantry. The whole concept of "being a noble" that I mentioned in the beginning possibly starts structuring itself around the end of the X century and throughout the following decades. Especially in the closing years of the 900s, the clergy appeared to put a great amount of effort into convincing the warrior aristocracy that the Christian ideals and the church were core elements of the condition of nobility. Principles such as piety, charity, mercy for the weak and the repeal of violence (yes, it's a complicated matter) were presented as elements of being warrior aristocrats. It is the same period where the ritual of knighthood starts to be integrated in the political structuring of the bannal lordships of the early XI century (privately owned land where a lord rules not by a mandate by a higher authority but just because he has the power to do so) and including religious elements such as the presence of a clergyman.

In this sense, the aristocracy identified one of the core elements of their lifestyle in the martial practice. War and hunting were considered the proper activities for a noble and then for a knight. War included the mock battles of the early iterations of tournaments, which often resulted in as much injury and death as a battle proper. The fact that nobles make war their trade so to speak constitutes I dare say the vast majority of such ethos. An ethos which was expanded and treated in the many books and novels written in verses by the Occitan troubadours of the XI and XII centuries. Epics such as the Matter of Britain (the Medieval literary tradition around king Arthur and the Round Table) and the Matter of France (the same but regarding Charlemagne and his paladins) seemingly included the qualities of knighthood and aristocracy, which influenced or were influenced by the courtly culture of European monarchies of the XII-XIV centuries, including the notion of courtly love.

This must have been clear to the contemporaries as well, since a prominent German aristocratic bishop, Adalberon of Laon (ca. 950- ca. 1031) wrote of an ideal division of the society envisioned by God: the laboratores ("those who work", the peasantry), bellatores ("those who fight", the aristocrats) and the oratores ("those who pray", the clergy, the most important ones according to Adalberon).

The peasants were considered to be inferior to the nobles. One of the main differences was possibly linked to the aforementioned military craft, in the sense that the only "professional" warriors for the majority of the Middle Ages were in fact aristocrats who could afford not to work in order to pursue military matters and military activies such as horse riding and fencing. Peasants were not ideally meant to fight as their first prerogative. If not because they didn't know how to do so as much as the nobles did, a thing which however didn't prevent them to go to war anyway (and be poorly considered there as well), peasants were associated with a condition of dullness and other negative traits. Some have hypothesized that the English word "villain" comes from the Old French term villein, "the inhabitant of a village".

Even in a context of very different social mobility like the city-states of Center and Northern Italy like Florence or Milan, the urban nobility (which shared almost all the trait of their rural counterparts) had to share or even lose power to very wealthy merchants and busisnessmen who managed to wrestle political influence away from them and toward themselves, creating a situation where immensely rich bankers were more important than aristocrats with century old lineages, such as the case of Florence, where the oligarchy who ruled the city was gradually overtaken by the aforementioned craftsmen and merchants who then created a set of laws called Ordinamenti di Giustizia ("Ordinances of Justice") between 1293-1295 who prohibited to all families who had at least one noble or knight ancestor from holding any sort of public office. This was done to weaken said families, whom many feared would simply arm themselves and violently respond to such decision because, yet again, peasants were peasants no matter how wealthy they were (of course, over time old aristocrats married into these wealthy merchant families for a number of reasons but that's another kettle of fish).

In conclusion, nobles did consider themselves apart from the rest of society and considered themselves better than the peasantry, but the peasantry was never treated as slaves or disposable workforce. I hope this brief overview helps your inquiry. Unfortunately, it's a very broad and dense subject.

Sources:

Provero, L., Vallerani, M. 2016, Storia Medievale, Mondadori Education, Roma;
Ruggiero, G. 2014, The Renaissance in Italy: A Social And Cultural History Of The Rinascimento, Cambridge University Press;
Cardini, F. 2006, Il guerriero e il cavaliere, in Le Goff, J. a cura di, L'uomo medievale, Laterza Editore, Bari;
Delle Donne, R. 2012, Burocrazia e fisco a Napoli tra XV e XVI secolo: la Camera della Sommaria e il Repertorium alphabeticum solutionum fiscalium Regni Siciliae Cisfretanae, FedOA Press, Firenze;
Rivera Magos, V. 2021, I conti erariali dei feudi nella I serie delle Dipendenze della Sommaria dell'Archivio di Stato di Napoli (XV secolo) per un nuovo inventario aggiornato, in Senatore, F. a cura di, La signoria rurale nell'Italia del tardo medioevo. Archivi e poteri feudali nel Mezzogiorno (secoli XIV-XVI), Firenze University Press, Firenze;
Albertoni, G. 2015, Vassalli, feudi e feudalesimo, Carocci Editore;