One of the best non-technical overviews I have found on the Internet is Papers for printmaking at the ARTTalk website. On a somewhat more detailed level, the Wikipedia article on paper, as well as articles on cotton paper and sizing, provided most of the following, though I've paraphrased and summarized.
Paper is produced by pressing together moist fibers, usually cellulose pulp from wood, grasses, or rags, and then drying them into sheet form. The process is thought to have developed in China during the early 2d century AD, though archaeological fragments of paper have been found in that region that date back to the 2d century BCE. China and the U.S. are the leading producers of paper today. Most of the printmaking paper I've seen is called "cotton rag", or "rag paper". Cotton paper is stronger and more durable than paper made from wood pulp, which can also be acidic. From the Dick Blick website:
The distinguishing feature of a good printmaking paper is its ability to take a soaking, to absorb a lot of ink, often with multiple runs through a press, without disintegrating or deforming. Fine printmaking papers are made with an archival fiber source, which in the European printmaking tradition is usually cotton. Japanese papers are often made with kozo (mulberry bark), another durable and archival fiber.To express thickness, paper is gauged by weight. The U.S. uses a system expressed in pounds, derived from the weight of a ream (500 sheets) of a particular paper cut to a particular, undefined size. Photocopy paper, for example, is somewhere around 24 lbs in weight (obviously not the weight of a single sheet, nor even of a ream of 8-1/2"x11"!). But in the printmaking world, paper is weighed according to the International Standard for paper sizing (ISO 216) used most everywhere else in the world. This weight is in "grams per square meter", expressed as g/m2, gm, or just g, and thus ties a specific weight to a specific size. That 24-lb photocopy paper is 90 g/m2. Paper for normal printing is generally between 60 gm and 120 gm. As indicated above, printmaking paper is much thicker.
In order to regulate how absorbent a paper is, sizing chemicals are usually added during the papermaking process to protect or glaze the fibers and force ink or paint to coat their surface rather than penetrate. Starch and gelatin are two examples of sizing substances. (Paper sizing has nothing to do with the physical dimensions of the paper; the term comes from Old Italian sisa, a glue used by painters to prime canvas or paper.) Paper that contains no sizing is called "water-leaf". Blotting paper is water-leaf, which is why it is so absorbent. Almost all other paper in sized internally during the manufacture process, and high-grade bond and writing papers may also be sized with a surface film that makes them very smooth and somewhat water-resistant. The amount and type of sizing used affect the archival quality of the paper.
Printmaking paper needs to have some absorbency, so it can't be highly sized. The convention, in fact, is to soak the paper for some period of time (I've seen recommended anywhere from 20 minutes to 24 hours), to loosen up the sizing and allow the paper to absorb more detail from the plate.
For now, this seems to satisfy my curiosity.