These knives, as the name implies, are designed to hang around your neck. They will generally feature 1.5″ to 3″ blades. The handles will generally hang down. However, there are some knives worn around the neck with the handle going up towards your chin. How you wear the knife around your neck is up to you.
Neck knives will more times than not feature a Kydex sheath. This is due directly to the weight and retention ability. Leather sheaths will stretch over time. This could lead to your neck knife not being as secure as you would like it to be. Not the scenario you want for your neck knife.