The Snark is a special tool that is utilized by Medics in the NeoTF game mode. These little bug like creatures can seek out enemies and attack them, whittling away their health and allowing the Medic or another teammate to deal a killing blow.
The basis of the Snark was seen in Valve's debut title, Half-Life, where these creatures were used as little attackers against aliens. Their appearance in Team Fortress Classic may have been due to the NeoTF creator being a fan of the game. Regardless, these little entities can scurry along the ground, attacking only enemies of the opposite team and doing damage in small doses. It is unknown whether a Snark can tell the difference between a Spy and a teammate, but many people believe they can't.
The Snark has its usages, one of them being a small annoyance, distracting enemies and allowing some team members to get through enemy lines. Since they do such small damage, the Snark is not considered a high quality weapons and are mainly used against Snipers, Spies, enemy Medics and Pyros. Unfortunately, Snarks also have a low damage tolerance, which means they can be destroyed easily.