Probably the most fertile ground for discord in any society is that of treating one person differently from another because of what is construed to be an invalid, unjust or unfair reason.
This treatment can refer to a person to person exchange, or between a person and an institution or society or government and on and on. Once you start down the path of discerning discriminations, it really is boundless.
There you quickly find yourself at the threshold of having to have a valid basis for deciding against someone about anything. This is usually a good thing but it can become unmanageable and disorienting especially when it's rarely discussed at any length or depth.
The "valid" in "valid basis" is a malleable word. It needs defining and examples. As does "construed" because that depends a great deal on who's doing the construing.
We're only going to scratch the surface although there may be a way to cut through the gordian knot of discrimination.
For any person in question regarding possible forms of discrimination, there are considerations about what roles that person is to play; what functions they are to perform; what authority they are to be to granted and are they able to grant; and what types of interactions & exchanges are they to be involved with.
These factors all involve treating persons in specified ways and usually affect decisions about how someone is to be considered or treated. Then if discriminations are to be consciously employed, valid reasons, if there are any, must be put out in the open. They are often couched in terms of personal or public safety questions, national security issues or religious or spirtual beliefs. The counters to discriminations work best when the distinctions in sight are unassailable like religious beliefs - how can anyone question religious beliefs? (That will surely come especially as some of the more egregrious tenets of various religions come to light in the Age of Internet)
How do we discriminate, let us count some of the ways.
By gender - only men can/should do this, only women can/should do that, only boys, only girls. It gets more graphic in some cultures/religions when menstruation is involved and prohibitions are invoked; or, genital mutilations (and this does include circumcision regardless of bogus health claims).
By religion - only persons of a given religion will be considered; only persons of a given religion will not be considered; persons belonging to smaller cult-like or "foreign" followings will be considered suspect.
By race - persons of a given race will be considered or accepted; persons of a given race will not be considered or accepted. In some cases, persons of a given race may not even be considered to be humans. They may appear human-like but by some magical stroke of illogic they are pseudo-humans, lower beings akin to dogs or horses, to be used by the master.
By age - persons of a given age range cannot vote; persons of a given age range can be drafted into the armed forces and forced to learn how to kill and be shipped off to wars they may not agree with or that may even be illegal. Persons of a given age range may be excluded from certain jobs or forced to retire/quit.
By sexual orientation - persons of a particular sexual orientation are accepted; persons of another sexual orientation are not accepted.
By physical appearance - persons who have deformities or handicaps are not accepted; persons who do not walk or talk or sit or stand a certain way will not be considered or accepted. Women whose breasts are too large or small are not accepted. Men who are bald or too short are not accepted. Or, no midgets or giants need apply.
By culture or ethnicity - persons who belong to a particular ethnic group or culture are not to be considered or accepted. In fact, we may actively seek to exclude them and they will be rejected if they somehow gained entrance to "our" group or company or club. Persons with the wrong names are to be excluded.
By language - persons who do not speak a given language at all or at a certain level of fluency are not to be considered or accepted. Persons who only speak a given language are to be excluded.
By wealth - persons who are poor are not to be considered or accepted; persons who are rich are to be given prefential access & treatment; persons who are rich are to be considered guilty or suspect. Only landholders can vote.
By citizenship - persons who are citizens of a given country are not to be considered or accepted; persons who are citizens of a given country are to be considered or accepted or given special treatment or access.
By genealogy - this was a bigger consideration earlier on with aristocratic bloodlines and the like; it ties in with race for some. All the "royal" houses carry this form of distinction - you cannot ascend to the throne now without the correct pedigree. The incumbents will search out for a suitable "outsider" to enrich the gene pool of a given blueblood line. This can lead to abusive forms of discrimination based on the humanity or lack thereof of different "persons" under consideration. Think slavery. Think genocide.
By social status - persons given or denied consideration or acceptance based on whether they are aristocrats, wealthy, famous, powerful. This would include types of caste systems and mandatory lifetime guilds based on crafts & skills.
By mental disability - persons given or denied consideration or acceptance because of perceived or real mental differences. In most cases, this is mainly focused on mental challenges that make one dangerous to others, but, often enough it's just being different that's the problem (It scares people).
By membership - persons given or denied consideration or acceptance because of their membership in a specified club or organization. One of the worries about this form of distinction is that some of these organizations are secret or have significant secret mandates; they often transcend national boundaries and laws and are usually not subject to any societal oversight. This would also include criminal syndicates and special extra-legal units of police, armed forces and intelligence services. And political parties both public and underground.
The fact that such and such a type of discrimination is less in force now than previously does not dilute the need for it being categorized. Any form can be revitalized in an instant given the right conditions.
The basic driving phenomenon is that anything that can distinguish one person from another can be used as a basis for discrimination. The discriminations we are most familiar with merely being the most obvious or easiest to detect or the ones that arouse the most fear or disgust.
Making distinctions is one of the primal talents of humans. Assigning judgements and emotions and attendant values and actions is typically when our problems ensue.
The countervailing human need is to be treated equally, justly & fairly.
The issue of discrimination is usually framed as a legal issue but it also reflects a society's stance even if that society also has many conflicting opinions within itself.
Society as a whole would consider that requirements for education credentials are a legitimate form of discrimination. But there are constraints on this requirement - Is the education credential being requested relevant to the job function? Can restrictions be placed on whose credentials are accepted?
Some of the types of bias that are, or were, considered acceptable: for/against childless couples; for/against families considered "too large"; for/against fashionable or unfashionable persons; for/against artistic persons; for/against people who are considered attractive or unattractive; for/against persons considered healthy or unhealthy.
One of the most difficult discriminations to address are those against what are often referred as issues of character although some have been re-categorized as mental health or genetic pre-dispositions. For example, is it fair to discriminate against someone who has an addiction? Does the type of addiction matter? Are there limits on how a criminal conviction can be used against someone? The admission of a problem is typically a mandatory step in being considered along with seeking help and adhering to a rehabilitation program.
Then there are the complications of times of war & conflict when patriotism and accusations of collaboration enter the picture. Everything gets boiled down to the ultimate simplistic discrimination: "for us or against us". It implies that the other discriminations have been surmerged by this new all-encompassing distinction but they always re-emerge after the conflict, sometimes during.
They never disappear because they are integral to the human makeup. It really is endless.
Wait until we encounter non-human sentient beings to see how powerful and resilent discrimination is and can be. There will be tsunami of it then.
Is there any way out or over, under, or through this morass? Perhaps.
The third Chinese Chen (Zen) patriarch, Hsin Hsin Ming, devoted a meditation/chant to the phenomena of discrimination. We'll end with some of his observations & admonishments as translated by the Zen Centre of Rochester, New York: "even slight distinctions .. set earth and heaven far apart", "discard opinions pro and con", "our choice to choose and to reject prevents our seeing this simple truth", "the more you talk and think on this the further from the truth you'll be", "this heavy burden weighs you down - O why keep judging good and bad?", "if mind does not discriminate all things are as they are, as One".
In the dogma of the marketeers, free enterprise and unencumbered trade is the elixir of prosperity and the guarantor of freedom. In reality, it is no such thing. 
