Tefillin are made from a single piece of kosher animal hide formed and dyed black, folded and sewn into small cube shaped boxes, batim, each containing tiny hand copied parchment scrolls with passages of Torah rolled tightly inside.
A specialist craft
The boxes and their leather straps, retzuot, are produced by specially trained artisans rather than a general leatherworker, since the stitching pattern, the exact black dye, and the precise folds are all governed by halakhic requirements, and a flaw in any of them can render the pair unfit for use. This makes a properly made set of tefillin a genuinely expensive object, closer in production cost to a small Torah scroll than to ordinary leatherwork.
Who wears them, and when
In traditional practice, Jewish men begin wearing tefillin around bar mitzvah age, thirteen, binding one box on the upper arm and one on the forehead each weekday morning, though not on Shabbat or festivals, which are considered signs enough in themselves. Egalitarian and some Conservative communities have extended the practice to women as well. The internal parchments are checked periodically by a sofer, and if found damaged beyond repair are buried rather than discarded, the same treatment given to any worn out text bearing the Divine Name.