In fortress mode, a peasant is basically an unskilled laborer, a dwarf who is at most of dabbling skill when it comes to labor. Peasants are automatically assigned only to hauling tasks, and may be important in this regard - a large body of haulers will keep your dwarves from dealing with the tedium themselves, allowing them to go about their work more efficiently. They appear teal on the unit list.
- Peasants appear in migrant waves. Note that military and social skills are not considered when labeling a dwarf - a peasant may well be a skilled military fighter, but still only haul around stones at the workplace. More often than not, peasants in migrant waves have some military skills and relatively few in-fortress relationships, and therefore make good drafting targets for your military. Note that peasants in the military that are sent off-duty have an additional negative thought from being relieved of duty; this can be solved by assigning them to an easily-trained or needed task (engraving, for instance) until they reach at least novice skill at it.
- A peasant is produced when children reach their twelfth birthday. Children that have been around a while usually gain levels in the grower skill, because "Harvest Plants" can be done by any dwarf, children included, if you kept the "Dwarves All Harvest" option in standing orders, but such skill is not reflected in their profession until they next gain a level (in any skill)Bug:1482. Children that come to full term will likely have lots of strong relationships around your fortress, and drafting them into your
suicide squads army can result in tantrums. Fortresses that manage to survive until their fifteenth year should expect to see plenty of Peasants and, like with today's youth, should be prepared by building lots of comfy white-collar workshops.
In many fortresses, peasants are forced to remain unskilled laborers by only assigning them hauling tasks. Peasants can be used to exploit the mood system. By assigning a peasant a skill which requires significant cost to the fort to train (i.e. any of the smithing skills, or glassmaking if sand or fuel are scarce) until he reaches a level of novice, then un-assigning the skill, the next time a stray mood is assigned, the likelihood of a legendary laborer in the critical field is heightened. Due to skill rust, peasants assigned in this way to the "mood lottery" can be reassigned easily later on.
Or you can simply use them to explore the caverns, draft them into the military, or... other things.