Glossary

I got into a bit of a debate with a touchy blogger primarily because he used a different term than I did and thus refused to see the point I was making so I thought I'd specially call out a few terms for clarity. For the first two I'll borrow the definition used by Bearded Devil as it very well written and matches my own.

Prepping - Providing a detailed, thought-through environment for players to explore and inhabit.
Planning - Aiming for a specific narrative arc or set of story beats.

The way I see it planning is the stuff of railroads, story games, and boxed text. Prepping is detailing the sandbox so the GM is ready no matter what madness the players come up with.

Judges Guild Approach - Another term I got from the Bearded Devil post linked above and it goes hand in hand with Prepping. This means providing as much detail as possible. Mapping as much of a city as possible (City State of the Invincible Emperor by Judges Guild mapped the whole city, thus the term) so the characters can go anywhere, do anything. This is the opposite of the Vornheim approach. From my own experience I'd have called this the Harn approach as back in the day the combination of the Harn regional module and Cities of Harn gave me latitude and confidence to let the characters run amok.

Vornheim Approach - Seat of your pants world creation at the table using imagination and tables. This approach is neither better or worse than the Judges Guild Approach. Different GMs will have a different preference of more likely they'll find middle-ground somewhere.

West Marches Campaign - Back in the day Gary Gygax seemed to let different adventure parties exploring Castle Greyhawk instead of having a set group of players and day to play. The different groups would clear out areas and steal treasure before another group explored a location. it was because of this initial style that he obsessed abit about time in the dungeon. Then Gygax went away from this style and moved towards tournament modules. The Knights Errant over at KnightsSemantic have redescovered this style of play and have had great success in the process. Its detailed in the post The West Marches: A Style of D&D Campaign for Large Groups. Even if you don't have a group so large that some are pushing for adventures in the middle of the weak the sandbox setup developed for the campaign sounds exactly like the kind of thing any GM should be aiming for.

Ars Ludi has more detail on the West Marches Campaign in Grand Experiments: West Marches.

Social Class: The Lower Classes

Townsman
A Freeman that lives in a town. Generally considered higher than a Freeman peasant because they have skills in crafts or are merchants.

Yeoman
Poor farmers that paid rent to work their land. Also known as a Freeman. As a culture gets more civilized the number of yeoman increases as the incentives of wealth for their efforts encourages harder work and greater crop yields than Serfs ever produced. A yeoman that moved to a town would be considered a townsman.

Serfs
Poor farmers that owe a debt to the lord and must work the Lord's land for him (might include mines as well). They can depend upon the Lords protection to an extent. The children of a Serf are also Serfs. After the black plague reduced the population of Serfs in England the peasants revolted resulting in a large number becoming Yeoman. A serf that escaped their lot and made it to a Freetown, and avoided capture for a year would be considered free and be considered a Townsman.

Slaves/Thralls
One that is owned by another. Slavery was common in the Dark Ages but mostly gone by the medieval period. The main difference between Slaves and Serfs is that slaves are protected because they are property, not because the owner has an legal requirement to do so.

Subinfeudation 101

England showing Historic Shires with Middlesex highlighted

Empire

Multiple Kingdoms or large regions under the same crown are typically known as an Empire. Examples would be Angevin Empire which at different times contained England, Wales, Scotland, Ireland, Normandy, Aquitaine, Brittany and other bits of France. Typically they function similar to a Kingdom with various King ruling it all acting as king of the bit and pieces as well.

Kingdom

At the next level we have the Kingdom.
All land in the Kingdom is owned by the crown. That's a lot of territory for one person to control so the crown subdivides everything into Duchies & Shires.

Duchy
A Duchy is basically the same as a Shire (below) except it is controlled by a Duke who is usually a member of the Royal Family and uses the Duchy as a vacation spot and money generator so they can enjoy life at court.

Shires
The term Shire and County are used somewhat interchangeably with shire being older and a bit cooler sounding so I'll use Shire for consistency sake. A shire is held by an Earl. William the Conquerer tried to bring the title Count over from France but it never gained traction. Oddly enough the Earl's wife is known as a Countess, probably because Earless sounded daft. The Shire boundaries throughout England haven't changed much until more modern times although some have been busted up a bit.
An Earl controls most of the land in the Shire (at least land not directly controlled or granted out to someone else by the King). The Earl may also control land in other counties as if he were a normal Baron in those areas.

High Sheriff
To keep a tight reign on things the King usually appoints a High Sheriff yearly. The High Sheriff had number of Sheriff's under their charge and would be responsible for enforcing law and order throughout the Shire (not so much collecting taxes despite some tales, tax collection was primarily the responsibility of the Duke or Earl). The High Sheriff was responsible for organizing the Shire militia, chasing outlaws (tales got that right at least), and dealing with court cases. The High Sheriff was often a commoner but had the power of the King behind them.

Hundreds

Middlesex hundreds with Elthorne highlighted

Each Shire was divided into Hundreds (or wapentake in areas conquered by Vikings). A hundred was an administrative unit for taxation and legal matters and doesn't have a titled holder. Originally a hundred meant that the area could support a hundred hides, or a hundred families, but that changed over time as populations increased and they never bothered to chop up the hundreds to keep up. Each hundred typically had a Bailiff appointed by the king who worked beneath the Sheriff.

Barony
A barony was the same as a Fief (below) except held by a Baron, one of the lowest of all hereditary titles. Being a hereditary title meant the Baron had the ability to pass property to descendants which meant in time a Barony was likely to expand to a number of fiefs through marriage and the blessings of the crown. Baronies were granted directly by the crown rather than by the Earl.

Fief
Each Hundred was subdivided into  fiefs. This isn't exactly true as the fief boundaries and hundred boundaries do not always line up but things get messy if you nit-pick. A feif was usually granted by the Earl to ensure someone loyal was watching the fiefs in the Shire (with the exception of Baronies which the Earl had little control over).

Elthorne Hundred with Harmondsworth highlighted

The fief was originally sized so that it had enough land to support and equip one mounted soldier. Better agricultural methods changed that later.

The Lord of the manor lived in a house known as a manor or manor house (over time the terms fief and manor became interchangeable). A manor might be fortified, in risky areas it might be a very simple Keep, but more often than not during the medieval period the manor was just a nicer houses than the peasants had.

Parish
The Church divided the Kingdom up into a subinfeudation of their own, but were kind enough to mostly duplicate the civil system. The Church used the term Parish for a fief. Each fief had a church to serve the needs of the worshipers in that fief. Over time the terms Parish and fief became interchangeable.

Social Class: The Nobility

King with globus cruciger and sceptre

King/Queen - The ruler of the land.
Prince/Princess - The children of the King/Queen. The Crown Prince, if there is one, is typically the heir to the throne.

The hierarchy of the remaining noble titles can be remembered with the mnemonic device:

"Did Mi'lord Ever Visit Bosworth Battlefield?"

Duke/Duchess - The lord ruling a Duchy or one granted the title ceremonially. Either way they are usually a member of the extended royal family. Beyond being ruled by a Duke a Duchy is like a Shire.
Marquis/Marchioness - A Marcher Lord. Similar to an Earl but with more power because of the risks required subjugating the Marches.
Earl/Countess - A baron that controls a Shire on behalf of the King/Queen.
Viscount/Viscountess - Similar to a High Sheriff but working for a Duke instead of the King. Originally non-heridatry the Viscount became a hereditary title over time.
Baron/Baroness - A hereditary title, typically the holder of one or more fiefs.
Baronetcy/Baronetess - A hereditary title but not a Peer (and thus not entitled to attend the House of Lords).

Well equipped Men-At-Arms

Beneath the hereditary Lords are:
Knight - One granted the honorary title of knighthood by a king/queen, bishop or other political or religious leader for military service.
Man-At-Arms - A term that applies to all military folks, typically mounted and well equipped, despite their title. A Mercenary peasant and Countess armed for battle could both be called a Man-at-Arms despite title or gender although in medieval history female Men-At-Arms were more than a bit rare.