A tablet might be defined as a physically robust writing medium, suitable for casual transport and writing. See also stylus.
Clay tablets were flattened and mostly dry pieces of clay that could be easily carried, and impressed with a (possibly dampened) stylus. They were used as a writing medium, especially for writing in cuneiform, throughout the Bronze Age and well into the Iron Age.
Wax tablets were pieces of wood covered in a thick enough coating of wax to record the impressions of a stylus. They were the normal writing material in schools, in accounting, and for taking notes. They had the advantage of being reusable: the wax could be melted, and reformed into a blank. The custom of binding several wax tablets together (Roman pugillares) is a possible precursor for modern books (i.e. codex).
The etymology of the word codex (block of wood) also suggests that it may have developed from wooden wax tablets.