Kerf marks are marks left behind by a bladed instrument like an axe, saw, or knife. Often, kerf marks form a very distinct pattern, which can be useful from a number of perspectives. In home design, the marks are sometimes used to create a feeling of hand-hewn planking, and they may be left intact on antique furniture to emphasize that the furniture was made by hand. In criminology, the marks are used to learn more about the weapons used to commit a crime, and sometimes these marks are so unique they are almost like fingerprints, becoming a key aspect of a case.
Any bladed instrument is going to leave some sort of mark as it cuts through material. The type of kerf mark created varies, depending on factors like the material the instrument is made from, the type of blade, the weight behind the blade, and how the blade is used. For someone who is familiar with studying them, these small marks can tell a fascinating story about a series of events, and they can be used to reconstruct a scene.
In criminology, for example, kerf marks can be used to narrow down the type of weapon used to commit a crime. Knives, axes, saws, and other weapons all leave unique signatures behind. Hand-held saws and power saws behave differently, while serrations of different widths leave markedly different signatures behind. Someone who hesitated might leave a false start, a partial cut next to the completed cut which can be used to generate even more information about the weapon used.
On television, it seems like criminal investigators are always uncovering kerf marks and miraculously tracking them to specific weapons. In fact, this doesn't happen very often, but sometimes it does, thanks to the efforts of researchers who have devoted their careers to the study of these marks. More frequently, forensic examiners generate a list of weapons which could have potentially been used, giving investigators more to work with.
While it helps to have years of experience to study kerfing, sometimes you can find examples around your home and garden which might be interesting. You may also have noticed that different types of saws behave differently when you use them, and if you examine the ends of cut wood after sawing them, you can see the kerf marks you left behind, often along with false starts, if you aren't familiar with using a saw.