Postage due is the term used for mail sent with insufficient postage. While the problem of what to do about letters not paying the full correct fee had existed since the creation of regular postal systems, it was greatly heightened by the advent of postage stamps, since customers were now making their own decisions about the right amount to pay, without the assistance of a presumably-knowledgeable postal clerk.
While at various times some countries have simply adopted the expedient of returning the letter to the sender, many others have taken the approach of delivering the letter and collecting the fee from the recipient. Initially the process was handled by a clerk writing something like "Due 3 cents" on the cover, but this was subject to abuse by mail carriers, who might write it on themselves and pocket the difference.
The problem was solved by France in 1859, with the issuance of official postage due stamps, affixed at the delivery office before being taken out to the recipients. Many other countries followed suit. (Occasionally, regular postage stamps have been used to fulfill a postage due function.
Postage due stamps (or "labels", to clarify that they have no value of their own) are not always affixed to individual letters; in the case of business mail, the total due might be summed, and the appropriate stamps added to the top letter in a bundle, or to a bundle's wrapper. The labels have also been used to collect money for other purposes, such as magazine subscriptions.
Since postage due stamps are almost always used only within a single country, they are usually quite simple in design, mostly consisting of a large numeral, and an inscription saying "postage due", "porto", etc; often there is no country name. As with definitive stamps, a variety of values are needed to make up specific amounts.
While technically there is no reason for postage dues to reach private hands unused, postal administrations have always been willing to sell them to collectors, and the postage dues of many countries exist in vast numbers, almost all unused and of negligible value. Valid usages on cover are much less common.