Serfs may be a more accurate term for these people. Many live in castle towns where they must give the few goods they can produce to the vassal, and receive a fraction of the product’s worth in payment. It’s not uncommon for peasants to resort to banditry and piracy.
It’s fairly easy for them to pursue these professions, as well. Vassals rarely keep close tabs on the peasants that live outside their castle walls. Just as you wouldn’t notice if a couple of bees were missing from a hive, it’s unlikely anyone other than another peasant would notice if one to two of his peers went missing.
Players with a peasant character may not take a character class that requires higher learning, such as wizards, and most other arcane classes, as their first class. Sorcerers and Warlocks are still available to them, since these powers are obtained through natural ability, or a pact, respectively.