Laymen are the everyday commonfolk that populate towns and are the force behind much of the world's economy. While most laymen are poor or modest in finances, there are some (especially merchants) that earn a comfortable or even luxurious living, but lack the noble blood to be defined as true aristocrats.
Farmers make up a large, if not the majority of, the commonfolk population. Often nestled far off in the countrysides and near-wilderness, but not unknown in towns and sometimes by cities, farmers tend to the land they live on. Producing crops and livestock for food and trade, they are just as important to the economy as any artisan or local baron. Most farmers are relatively poor but make a fair living off their crops. Some do find a scant amount of wealth beyond what they need to survive, but it is not common.
Merchants are the arteries and veins from which the lifeblood of trade and commerce flows. They sell their wares to whomever is willing to buy, and many find fortune in their work. Not all merchants are rich, however... Many are just as poor as any farmer and work only on local levels. Most merchants travel at least some distance between towns and cities, as their careers require the exchange of goods. More powerful and influential merchants may travel as far as overseas to maintain their wares, and may have several lesser merchants working under them.
Thieves are the pickpockets, bandits, and rogues of the world. They are most renown for their quick fingers, silver tongues, and prowess at weapons requiring dexterity. However, not all thieves are criminals; some are in the employ of governments to work as spies and underground police; others are justice-seekers, just by their own terms. Some even use more heavy-handed weapons, or are less skilled in speechcraft than most would first think. Whatever the case, it is best to keep valuables secured while in their presence.
The commonfolk that don't follow under the other specified groups are known as townsfolk -- and the term here is also used to describe the everyman in cities as well. They hold various jobs not clarified here or in the other class sections, such as bartenders or tanners. They are often poor or middle class, but some are members of higher society without the actual blue blood. Doctors and lawyers, for example, tend to have a larger income that comes along with their careers, but there are those who are of more modest assets.