The “Tang” is the steel part of a knife that extends back from the blade into the handle. There are two types of tangs, Full Tang and Stick Tang.
Full tangs extend back and follow the shape of the handle, with the scales (wood, plastic, etc.) sandwich it on either side.
Stick tangs (also known as hidden tangs), on the other hand, are fairly thin rods that extend back, sometimes all the way to the pommel.
Whichever way is chosen by the maker generally depends on style of knife, and types of handle materials. I personally prefer to make stick tang knives more, as my favorite knife styles are bowies, which generally look better as a stick tang construction. Technically, stick tang construction is weaker, which leads many people to the misconception that stick tang knives are weak. Let me explain. Full tang knives more or less have the strength of a bar of iron, more than enough strength to do general cutting jobs, including hacking through branches. Well made stick tang knives are weaker than a bar of iron, but not nearly enough to make them likely to break when doing heavy cutting jobs. This may seem counter intuitive when you look at how thin the tang seems, but remember that most of the stress and pressure (the place most likely to break) is only about a centimeter back from the blade, and so the amount of leverage (if you remember physics, if you increase the amount of force but decrease the length through which it acts, total force on the lever does not increase) is miniscule. Stick tang knives can hold up to any task full tangs are designed for, with strength to spare.