Dabs or dabbing refers to the act of holding a highly concentrated cannabis extract (such as THC or CBD rich oils) near a surface heated with a torch lighter and inhaling the vapours through a special type of bong or pipe called an 'oil rig'. Some have called dabbing the technological evolution of smoking cannabis.
The tool one uses for dabbing is simply called the dabber, which is typically a glass or titanium wand that allows you to take a tiny bit of concentrate (e.g. oil or wax) and place it on the so-called oil rig. The oil rig looks quite similar to a glass bong but it's especially suited for smoking (oily) concentrates.
The part of the oil rig where the dab is applied to and subsequently turned into smoke is called the nail. Nails come in several types: with or without a dome. A dome sits on top of the nail and keeps the smoke in. Domeless nails hold the smoke through an outer disk or holes around the nail itself.