How is serfdom related to slavery?

There are many forms of unfree labour. Prison labour for instance or forced or bonded labour. You might want to ask your question on Phil.SE if what you are interested in the philosophical distinctions after stripping away concrete historical conditions.

A serf is a form of bonded labour. He himself is not property. Whilst a slave is property. This is the basic difference.

One problem, as has been pointed out above, is that the terms "serf" & "slave" are largely defined in context of the culture they are used in. Almost any statement one could make about a "slave" could often apply to a "serf" too. For example, servants in ancient Rome were always slaves, yet (if I remember my Tolstoy correctly) Russian nobility had "house serfs". And while slavery can be found practically world-wide (I haven't encountered any examples of slavery in China or Japan, but I assume that is due to not looking hard enough on my part), serfdom is not unique to Europe: pre-modern Ethiopia had a system similar in many respects to serfdom (where they were called gabar or zega) -- as well as slavery.

But in general, serfs were unfree members of a feudal society; if there is no feudal, or feudal-like structure (experts argue whether pre-modern Ethiopia truly was a feudal society), there are no serfs. (AFAICR, serfs were owned by nobility, not by non-noble individuals like wealthy peasants or merchants.) Slaves were/are people who are property, like livestock. Because they are not free, slaves can appear in surprising contexts: for example, in some medieval Islam countries, entire armies were staffed by slaves. And while a runaway serf could be punished quite harshly when caught, the penalty for a runaway slave in ancient Rome was usually death. (Augustus in his Res Gestae boasts about the number of runaway slaves he crucified.)

The distinction between the two is a complex subject, & I'm disappointed that the Wikipedia article is so superficial about the nature of the unfree class of people.

Most of the answers focus on the similarities between serfdom and slavery, and it can often get subjective based on the one's political views (an anarchist or communist might claim that modern salaried employment is the same as slavery, because the employee doesn't own the means of production and most of the profit goes to the employer).

Therefore, here are the main conceptual differences between slavery and serfdom:

A serf is basically renting a piece of land from the lord, where he can grow his own produce for himself, and he pays the rent in a share of the produce and/or with a specific number of days per year having to work for free for his lord. The difference between him and a free landless peasant is that he is not allowed to terminate his rent contract. But he is not technically the property of his lord. The lord might sell that land to another lord, and then the serf will become the subject of the new owner, but he stays in the same house with his family and will continue to work the same plot of land.

A slave, on the other hand, is basically like cattle, being bought and sold individually. Usually cannot freely marry, and the slave's children will often be sold. A slave cannot really make long-term connections with his community, because he might be sold at any moment, being transported to his new workplace and might never see his old home/family/friends ever again.

A serf, on the other hand, can have a family of his own and raise his children on his own.

A slave usually has no personal property, everything a slave produces is the property of the slave's owner.

A serf, if working harder or being more lucky, has the chance of having better food or building a better house or having better tools for himself and his family, than other serfs who don't work as hard.

In English Law (post Norman-conquest) there are in general two classes of Villeins, with shared aspects:

• Villeins en regardant:

  A villein annexed to the manor of land; a serf.

  The consequence of being annexed to the land is that these villeins are both required to, and guaranteed the right to, till the land to which they are annexed and enjoy its produce provided they perform the specified number of days (usually about 40) labour for their lord each year.

• Villeins en grosse:

  A villein who was annexed to the person of the lord, and transferable by deed from one owner to another.

  Such villeins could be bought and sold independently of the land they tilled.

  Cobbett's Complete Collection of State Trials and Proceedings for High ... notes (pp 42-43):

  The author of the **Mirroir* (c. 2 S 38), who wrote in the reign of Edward the 2d, only mentions Villeins regardant: and sir Thomas ASmith, who was secretary of state inj the reign of Edward the 6th says, that in his time he never knew a villein in gross throughout the realm (Smith's Commonwealth, b. 2, c.10.). However, after a long search I do find places in the yearbook, where the form of alleging villeinage in gross is expressed, not in full terms, but in a general way: and in all cases I have yet seen, the villeinage is alledged in the ancestors of the person against whom it was pleaded, and in one of them the words 'time beyond memory' are added.

Common to all villeins is that their status as villeins is only in regard their lord, and to no other person in the realm, but that to all those others they are regarded as freemen.

Again from Cobbett's:

The [plea of bastardy] was an allegation by the supposed villein that either himself or his father, grand father or other male ancestor, was born out of matrimony; and this plea, however remote the ancestor in whom the bastardy was alleged, was peremptory to the lord: that is, if true it destroyed the the claim of villeinage, and therefore the lord could only support his title by denying the fact of bastardy.

Note that in addition to the classification above of villeins by the terms of their socage villeins were further classed, independently, by terms of their residence arrangements and/or the size of their land right:

Pollock & Maitland's The history of English Law before Edward I provides even more background by searching on "villein".

Slavery and serfdom are legal terms; every legal framework (roughly every country) will have different definitions. So as you point out Roman slavery is different from French slavery, and both are different from serfdom.

There is a linguistic principle that the definition of words refers to a cluster of related concepts (think of all the objects that can be described as "chair" - office chair, Danish modern chair, classroom chair, comfy reading chair, etc. If I were to use the word "chair" to refer to a barstool or another object on which someone were sitting, few people would be confused. The meaning of "chair" depends on the context in which it is used. After I wrote this, you provided a similar example - think of all the creatures that we refer to with the word "dog", from chihuahua to great dane)

For slavery and serfdom, the specific meaning depends on the context in which it is used, but the general meanings are as described in Wikipedia.

• Wikipedia:Serfdom

  Serfdom was the status of many peasants under feudalism, specifically relating to manorialism, and similar systems. It was a condition of debt bondage and indentured servitude, which developed during the Late Antiquity and Early Middle Ages in Europe and lasted in some countries until the mid-19th century.

  @o.m points out something that I missed, and I can't say it better than o.m did, "specific and mutual obligations between the serf and the lord at many times -- e.g. the serf owed the lord X days of tilling the fields, but the lord owed the serf a beer ration during those days. The lord could not simply send the serf into a workshop, he only had the right to demand field work" Serfdom is a mutual relationship with obligations on both sides. In most forms of slavery, the master has no obligation to the slave.

• Wikipedia:Slave

  Slavery is any system in which principles of property law are applied to people, allowing individuals to own, buy and sell other individuals, as a de jure form of property.

Note that both reference a legal framework - serfdom references feudalism (technically an economic framework associated with a set of legal frameworks, but the distinction probably isn't important for your question). Slavery is specifically references as "de jure".

Slavery permits people to be considered property. Serfdom is servitude in which the serf is still recognized as "person" not property.

The distinctions between the various types of slavery or the various types of serfdom have to be studied each individually. If you want to understand Roman slavery distinct from American slavery, you have to study the laws and jurisprudence that define each framework. Similarly, if you want to understand the distinction between Russian serfdom and English serfdom, you need to study the laws and rulings that define and govern each.

Hope that helps somewhat - I admit that it is less an answer, and more an admission that the answer is difficult.

You asked whether there is a fundamental difference between the two - depends on the context. Is there a difference between a hound and a dog? If you're looking for a pet, then no. If you're going hunting, then quite possibly. If you're a serf, you are going to be passionately committed to the notion that you're not a slave, and if you're an enslaved person, you're going to be persistently aware that you're not a serf.

If you can buy and sell a person, you're talking about slavery.