Print descriptions should be comprehensive and clear. This is particularly critical in the internet buying context, where the print can't be seen in person. Here I have given a description of the print shown above, and after many of the terms describing the print I discuss their meaning. Here goes:  

Adriaen Van Ostade (1610-1688), The Singers, circa 1668, etching. References: Godefry, Hollstein 19[these are catalogues in which the print is described in detail; the number refers to the number they give the print].

The fourth state of seven [state - each time the print matrix, in this case a copper plate, is changed and an impression of the print is taken a new state has been created]. For a more detailed illustrated discussion of fine print states you might want to see this guide: Fine Prints - States of the Art

In very good condition [A good seller is a bit conservative in describing condition, so buyers are happily surprised. All flaws should be noted - tears and nicks, staining, holes, foxing, rubbing, ink marks, repairs, prior hinging, whether laid down or glued to a backing, creases, folds, etc.]

With the inscription on the plate A. v. oftade fecit et excud [these words, printed, indicate that Van Ostade made and published this print. Prints were not signed in pencil until two centuries later.  Print descriptions should of course distinguish whether prints are signed in the plate or by the artist in pen or pencil. One way of doing this is to note the letters in the plate in brackets, and also to add that such letters are in the plate, so no reader will be misled.]

With margins, 9 7/16 x 7 15/32, the sheet 9 3/4 x 7 3/4 inches [the first measurement shows the size of print measuring the plate mark, height preceding width, the second the size of the sheet overall]

On old laid paper with the Fleur de Lys in Crowned Shield watermark, a variant of the several comparable watermarks cited by Godefry as characteristic of the early impressions [old prints, before about 1750, were made on laid paper, with lines indicated the wires on which the paper was dried; the watermark here is further evidence that the print was made early in the plate's history]

Archival mounting (acid free hinging on acid free board and window mat, glassine cover) [this shows how the print is currently stored; fine prints are not generally sold in frames, and of course proper inspection requires that they be out of the frames. In museums prints are stored in boxes, in mats].

Provenance [this describes the prior ownership of the print, if it is known] J. Danser Nyman, sale Amsterdam, March 19 1798, to Hendriks at Knoedler Gallery, New York (Lugt 2007) [this is a reference to Lugt, which describes and identifies the stamps owners often applied to the back or front - verso or recto - of prints]; Dr. S. W. Pelletier (with his stamp verso, not in Lugt) (This is the earliest state impression of this print collected by the eminent Van Ostade collector Dr. S. W. Pelletier.)

A fine richly printed impression [impression quality -this can range from very fine to fine, very good, good, fair, or worse. It's a bit subjective, but few old masters are very fine, and very good is often quite good. In general the earlier the life of the matrix (here, copper plate) the better the impression] in black ink with plate tone [ink left on the plate when it is printed, creating a veil of tone or greyish quality where the ink has been left], with superb contrasts - a fine example of Van Ostade's use of chiaroscuro - on ivory laid paper.

This state shows Van Ostade's signature bottom right, and the three very tiny vertical strokes across the border above the jug at the top, before the further shading above [this indicates how the state was determined; all these changes from state to state are described in the catalog].

It is of course only in the early lifetime impressions that Van Ostade etchings can be fully appreciated. [an impression made during the lifetime of the artist, often printed by the artist personally. Even during the course of a lifetime artists may have their prints made at different times, so the earlier proofs tend to be better. Lifetime impressions are generally approved in some sense by the artists; posthumous impressions of course are not. Lifetime or not is a difficult judgement to make in many cases, but in this instance it's known that the artist changed or revised the plate several times, so the intermediate states are lifetime; it's also known that the plate was revised by others after his death, so prints exhibiting these revisions and re-workings can be identified rather easily as posthumous].

One final point - print connoisseurship is a complex matter, and even experienced print collectors will have questions about a print or its description. The cardinal rule to follow to increase your understanding of particular prints or prints in general is to ask questions of the seller. Good sellers will know the answers or try to find them, and will be happy to discuss problems of connoisseurship with you.  

For a good glossary of fine print terms I'd recommend the eBay guide: Glossary of Print Terms. I've also written a number of guides on fine prints, including a discussion of Rembrandt prints, print states, signatures, and other issues of print connoisseurship. Here's the link for these guides: Fine Print Guides

Thanks for reading this, and please feel free to contact me with your questions - Harris Schrank