In the modern western world, class is the primary form of social stratification. In a class system, social status is primarily shaped by one's wealth and income, which then dictates one's status as part of the lower, middle, or upper class. One of the key differences between a class system and a caste system, however, is that a class system is open to social mobility. There is the potential for individuals to ascend the class system, rising from a lower to a higher place in society. Likewise, the same applies in reverse: just as it is possible to advance in society, it is also possible to descend, falling from elite status into poverty.

A caste system, on the other hand, is a far more closed system, based entirely on social status and one's role in society. A caste is something one is born into, and in this sense, it defines one's place in society and even one's identity. Additionally, whereas in a class system, it is wealth and income that serve as the primary engines that determine one's status and prestige, the same does not apply to caste. Regardless of whether a member of the religious caste might be wealthy or poor, this would not change their identity as part of the religious caste, nor would it change the privileges and status associated therein. One peasant can be a prosperous landowner in their own right while another can be living in poverty and barely on the edge of subsistence: both would still be defined by their status as peasants.

Finally, slavery is defined by a concept of ownership, by which human beings are reduced to the status of property, owned by another person, and able to be bought and sold. From this perspective, where class and caste presents a hierarchy that organizes society as a whole (with differing levels of prestige and status connected with different levels on the hierarchy), slavery should be understood as something different altogether. Even with an extreme example such as that of the Ottoman Empire, which featured the Janissary Corps, personal slave-soldiers answerable only to the Sultan (and commanding vast wealth and political power), slaves are not permitted genuine self-autonomy but are entirely beholden to the will of another.

These different systems generally have some distinctions. Slavery in the United States, for example, became based on race. While some whites were poor and worked as indentured servants in colonial America (this also occurred in the years of the early Republic), they always had the possibility of freedom. However, slavery offered no hope of release, and slaves in the US were all African American. In other societies, slavery may not be based on race, but it generally offers no hope of freedom and involves the ownership of one person by another. In some societies (such as in the US before emancipation), slavery was inherited, but in other societies, it is not inherited.

The caste system is generally inherited. People are born into the same caste as their parents. This system may not necessarily be based on race. People generally cannot escape their caste but must live in their caste for their entire lives.

A class system is based on socioeconomic status. To some degree, this system is inherited, as it is hard to be upwardly mobile if parents do not have advantages that they can pass on to their children. However, in theory, a class system offers some degree of class mobility, as people can move up and down in the system. While class systems can be partly or largely based on race, they can also have people from different races be at different points in the class system.

Slavery involves one person owning another, which means that the owner has total control over the enslaved person's time, body, and movements. In the ancient world, anyone could become a slave due to the misfortune of debt—many people sold themselves into slavery to discharge debts—or through being a member of a conquered nation. Often in the ancient world, slavery was a temporary experience. It was not predominately based on race.

In the antebellum South, slavery was based on racial designation and was lifelong, except in the rare instances of a slave purchasing his or her freedom or being freed through a will.

Today, a person is considered enslaved if they are expected to work or do what they are told all of their waking hours and cannot leave their place of employment.

A caste system, most often identified with India, determines a person's status in society from birth and explains this as divinely determined and fair, as it is based on what a person did in a past life. More generally, a caste system refers to a highly stratified, rigid, hierarchical society, where it is very difficult to move out of one's birth status.

Class is a looser form of caste. While birth confers privileges or pains, this system presupposes that people can move from one class to another based on attributes other than one's parents, such as the attainment of a higher education, wealth accrued in adulthood, interests, and accomplishments. A person in a class system lives within a hierarchy but can, to some extent, move up and down the class ladder based on merit.

This is a very difficult question for two reasons. First, all definitions are hard to come by and there will always be people who disagree. Second, definitions differ based on context. Here are a few points to consider.

First, a caste system separates people based on birth. Merits do not factor in. Moreover, not all societies have a caste system. So, this framework will not apply to all societies. India is the clearest example of a caste system.

Second, slavery is when one person owns another. There are no slavery societies today (though there is black market slavery), but functionally speaking there are modern day "slaves," because they are forced to do things against their wills.

Third social stratification simply speaks of the hierarchies in society. All societies have this.

If we take a step back and examine all three there is a lot of overlap from a functional point of view. Functionally, if a person is poor and works merely to survive in a foreign country where he or she is looked down upon, then his or her situation is tantamount to slavery or being part of  a lowest caste system. In the end, it all really about perspective.