The way of the world.
Some behavior must be legislated in order for society to function. We need to have tax regulations, traffic rules, bankruptcy laws, and trial procedures. But then there are things that should not need to be the subject of bylaws, statutes, or house rules: chewing gum is not to be disposed of on furniture or a floor; answering a telephone requires a pleasant demeanor; in a crowded parking lot one should park within the lines. In Hebrew, such basic, obvious consideration for others is called derekh eretz, which might be translated literally as "the way of the world."
A Slippery Concept
The concept of derekh eretz is slippery, because it is often used to describe ideas one could have expected to remain unspoken. The term can mean "common decency," as when the Rabbis inform us that the Torah is teaching us derekh eretz when it instructs us to greet others even before they greet us (Mishna Avot 4:15) or when it instructs us not to enter another person's home (or even one's own) abruptly (Babylonian Talmud [BT] Pesahim 112a). In this usage, derekh eretz is reminiscent of a rhetorical ploy adults often use on children--the first-person plural as a veiled instruction, inculcating a sensitivity for social expectations: "We don't do that," "We do such-and-such this way," implying that one is aberrant or, at least, impolite if one does otherwise.
However, the phrase can also have a sense closer to its literal meaning, something like "the way things work," or "the way it is." It is this sense of derekh eretz that the Rabbis have in mind when they teach us to invite or allow our elders or teachers with whom we dine to take food first (BT Derekh Eretz 7), or when they instruct us that we are judged in our home towns by our reputation but elsewhere by our clothing (BT Shabbat 145b). That's the way life works, they seem to be saying, and if you're wise, you'll recognize it and act accordingly.
One realm in which one needs to learn how things are done (but about which society is sometimes squeamish) is the matter of "the birds and the bees." As such, the phrase derekh eretz, is sometimes found as a euphemism for sexuality--in the admonition, for example, to use words to set the right mood before attempting to cohabit with one's wife, a practice described as derekh eretz that one can learn from the rooster (BT Eruvin 100b).
All Psychology and Anthropology, No Theology
As a behavioral principle, derekh eretz stands out in rabbinic literature for several reasons. First among these is its lack of explicit theological grounding. At least on the surface, it is entirely about how we relate to other individuals and to our society at large, without explicit reference to our relationship with God. When the midrash Exodus Rabbah (35:2) instructs us that a person should refrain from using wood from a fruit-bearing tree to build his house and calls that rule a lesson in derekh eretz, its primary concern seems to be ecological and economic. Or when the late Talmudic tractate Derekh Eretz Rabbah (one of two short tractates on the subject appended to the Babylonian Talmud) advises us neither to rejoice among people who are weeping nor weep among people who are rejoicing, it is apparently teaching us to be sensitive to the feelings of others and to care for our own reputations as well.