Before cameras were commonplace, there was no way to save the images of places visited, loved ones, events, etc.......unless you were an artist. Doing an oil painting was fine, but it only produced one. Enter etching, which was (partially) portable and could produce many copies.
Artists such as James McNiell Whistler and Joseph Pennell carried etching plates (more later) with them, especially when travelling, and could stop and scratch the image into the plate with a diamond point etching needle, recording what they were seeing. When they returned to their studios, they did the second half of the process, the printing.
An etching is made on a copper (more expensive but softer) or zinc etching plate, which is simply a sheet of metal the size of the finished etching, with a polished surface. Several could easily fit in a saddlebag or suitcase. On location, the artist uses a diamond-point etching needle to scratch (etch) the image into the etching plate. Back in the studio, he applies ink to the plate, puts a piece of paper on top, sandwiches these between two thick felt blankets, and puts them through his etching press, a finished "dry point" etching. Because the lines in the plate are relatively shallow and they get "mashed" each time the plate is put through the press, the plate may not be able to produce many good prints. Drypoint etchings are almost always printed with black or sepia ink.
The more complicated process is usually simply called an etching. Before etching the image into the plate, the artist coats it evenly with wax , or ground. He etches the image into the plate and submerges it in a large tray of nitric acid. The acid eats away the artist's lines, or image, which is where the ground has been removed. The remaining wax protects the areas where the ground remains. The longer the plate is in the acid, the deeper and wider the lines become. An eagle feather may be gently stroked across the plate to remove oxygen bubbles. Most artists will judge if the lines have become wide enough by not only examining the plate, but also feeling the lines with their fingers. When he is satisfied, the plate is removed from the acid, the ground is removed with turpentine, and a print,(an artist's proof), is made. The artist, if he approves of the result, then prints as many etchings as he wishes. If he is not satisfied with the result, he may cover the plate with ground again and deepen the image with his etching needle or add more detail, then put it into the acid again. Several artist's proofs may have to be made and they all get the notation in pencil at the bottom, "AP" or, in French, "EA". Because the lines are wider and deeper than with dry point etchings, it's possible to print more etchings this way, before the plate breaks down.
Whistler printed his own etchings, and because of the time and physical work required, his editions were small, often around 25 prints. Most ethical artists today print editions of 100-300. When editions are much larger, it may mean that the artist is very commercial. Artists who are especially prolific may create the etched plate themselves, then turn the printing, the manual labor, over to someone who specializes in doing it.
