Strictly speaking, trench art is a phrase that describes folk art created by soldiers who were stuck in the trenches during World War I. But trench art as a more broadly defined genre includes all sorts of art objects made during numerous military conflicts going back to the early 1800s, including items produced by prisoners of war.
Leaving aside the question of era, there are several generally accepted categories of trench art. Some of the earliest examples are wooden boxes made by French prisoners captured by the English during the Napoleonic Wars (1803-1815).
By World War I, prisoners on both sides of that struggle were engraving and carving everything from spent shell casings to soup bones, transforming them into poignant mementos and useful historical records of the war to end all wars. Some pieces are particular to certain battle zones. For example, Turkish prisoners were known for their beaded snakes.
Those soldiers who did not have the time or tools to engrave spent artillery shells in the trenches often brought them home, where they would be embossed, fluted, and flared. Engravings on canteens and mess kits were probably done in the field, as were paintings on helmets.
Other examples of trench art include letter openers and knives made from bullets and shells; presentation plates hammered and engraved from flattened casings; lighters formed out of enemy belt buckles; and inkwells carefully crafted from fuse caps.
Soldiers recovering from their wounds also made trench art, usually in the form of embroidered badges and belts. Similar pieces made by wives and girlfriends waiting for their loved ones to return home are also considered trench art.