Life for a peasant in the Middle Ages was, not surprisingly, more difficult than life for an aristocrat. A student once asked a colleague of mine, "Why would anyone choose to be a peasant? Was it a bad high school counselor?"
Um, no. You'd be born a peasant. Although most of us can imagine ourselves as medieval knights and ladies, in fact, let's face it, our ancestors were peasants, along with a good 90% of the population.
Basically a peasant was a country person, someone who farmed. Although in the modern US only about 3% of the population lives on a working farm, in the Middle Ages, before modern hybrid seed and fertilizers, a farmer would raise barely more food than needed to feed his family, meaning that almost everyone was out working the soil. As any modern farmer will tell you, it's very hard work, and you're subject to random, uncontrollable events like storms, drought, and insect invasions. Famine sat at the shoulder of every medieval peasant (as is still the case in much of the Third World).
A peasant might be completely free, owning his or her own land. Or a peasant might be personally free yet rent land from a landlord (probably the most common situation). Or he might be a serf. A serf was not a slave, as agricultural slavery had disappeared in the sixth century with the breakdown of the Roman Empire (click here for more on the end of imperial Rome), but he or she was considered "bound in the body" to a "lord of the body." This meant a serf could not give testimony in court or become a priest, as he or she was not considered to have the ability to make free-will oaths. On the other hand, not being a slave, a serf could not be bought or sold and was not subject to arbitrary commands.
In practice there were a great many varied statuses for peasants, including many varieties of serfdom, depending on location and whether one was a descendant of slaves or a descendant of someone who had commended himself to a lord of the body in return for protection and support. In a village, every family probably had a slightly different status and owed different rents to different people. All of this was kept track of orally, which meant that slippage was always possible.
Anyone who rented land owed rents, of course, whether a free peasant or a serf: usually some combination of money rents, rents in kind (a certain number of chickens or bushels of wheat a year), and days a week in which the peasant was obligated to work on the landlord's fields, raising food for the landlord. These rents were (theoretically) fixed, not a percentage of the crop. Serfs as a group generally (though not necessarily) had higher rents, and in addition they owed servile dues, generally a few pennies a year, more symbolic than anything--but humiliating if, as in many places, the serf was required to approach his lord on his knees, pennies on his head.
Although a serf might have both a landlord and a lord of the body, and one would think this would make life very difficult, in practice it could be an advantage, as a serf could use one against the other. Serfdom ended in France and Italy in the early twelfth century, although it lingered longer in Germany and Britain. Russia acquired serfs for the first time after the end of Europe's Middle Ages.
Peasant houses were very simple, at most a few rooms, generally built right up against the cowshed (this can be an advantage in the winter--cows keep you warm). Their diet was also very simple, mostly bread and whatever vegetables might be available. Meat was an extremely unusual luxury. (Click here for more on the medieval diet.)
(Click here for more on medieval peasant farming, and read my post here on the word 'feudalism' before you use it to refer to medieval peasants.)