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.