Warrior: Warriors are not the brightest crayons in the box. They are very good at killing things and taking orders. They are also very strong.
Paladin: Paladins are like warriors, only they uphold strong and righteous ideals and do not kill needlessly. Paladins are symbols of everything right and true. They are also smarter than their dull counterparts.
Mercenary: Mercenaries are the hired fighters. They fight for cash only. For some it doesn't matter what the cause is, but other mercenaries have feelings and fight only for just ideals.
Knight: There are a few Orders of Knighthood, based on your principals. However, knights make up the minority of the fighter class, along with paladins. Mercenaries and warriors are the majority of fighters, but they are a chaotic class. Knights are the orderly class, whether lawful good or evil.
Ranger: Rangers are loners. They are the mysterious, rugged men who save peasant people from the wolves and stuff like that. They live in a secluded forest and are pretty intelligent compared to other heroes.
Druid: Druids are the nature-lovers. They are like most magic users and clerics, but they have nature-oriented magical spells. They put on a dumb act sometimes but can build up massive energy and connect spiritually with trees and other plants. I put them under "forest folk" and not "magic-users" because many druids possess no magic at all, but the powers of herbs and such.
Wizard: Wizards are untrained magic users. They are infirior to the magi, who train their skill. Wizards often end up killing themselves with their raw power.
Mage: Magi (a single one is a mage or magus) are different from wizards. A wizard is a person who acquires magic and does not train themselves to use it. Magi train their skills and dedicate themselves body and soul to their Art. Magi are usually less accepted in society than the wizards because they tend to keep to themselves and their studies.
Illusionist: Illusionists have no real skill in magic other than to create what is not there. They might produce an imaginary feast, but no real food. Illusionists are usually show-wizards who make the special effects for plays and parties.
Cleric: Clerics are many things; they are fighters, magic-users, and healers. I put them here because they are most reknowned for their healing. Many armies employ clerics because they are warriors, magi, and healers all lumped together. They are don't have the fighting prowess of fighters, the advanced skill of magi, or the complete powers of healers, but if you want it all... Well, it's all here.
Healer: Healers don't fight. At all. They only have healing magic. They aren't considered magic-users because their magic is limited to healing. If there are any healers in an army, it is understood on both sides that you don't harm them. They do their job, just like the ones on your team do theirs. Anyone in an army who harms the healers is either executed, thrown out of the army, or shunned for the rest of their life, depending on the land.
Rogue: Rogues are theives personified. They sneak into homes and businesses and steal valuables. They are mainly thugs that are lacking in character.
Highwayman: Highwaymen are the romantic rouges. They waylay caravans and merchant trains laiden with goods. Most young women who dream of the handsome and rougish men who will fall in love with them on the road dream of highwaymen.
Monk: Monks use no weapons, but they are kick-butt fighters. They don't have to take vows of silence, but those who serve certain gods do. And yes, girls can be monks.
Priest/Priestess: I don't need to explain this class. However, I'll note one thing: they use god-given magic, if provided.
Bard: Bards are the musicians. They usually carry lutes, flutes, mandolins, or lyres, but sometimes violins or viols. It is an unwritten law that, like healers, you cannont attack them unless they attack you first.
Pirate: Pirates are the villians of the seas, waylaying merchant ships and stealing their goods. Need I say more?
Slave: Slaves are the unpaid labor for a figure. They are usually freed or escape, and then they turn into one of the previous classes.
Servant: Servants are paid for their work or are given benefits, such as payment in food or shelter and the like. They could work in nobles' manors, in the castles, in the shops, or anywhere else.
Merchant: Merchants are the salespeople. They might travel in a caravan or own a shop. Either way, they offer goods that are critical to the survival of all classes.
Noble: Nobles are those of higher birth and royalty. They are the kings/queens, princes/princesses, dukes/duchesses, and all other lords and ladies. They generally own large estates or live in castles.