A grapheme is a unit consisting of one, two or three letters. A grapheme signals or represents a phoneme. Here are some examples of graphemes in English: <sh, th, a, qu, r, tch, oi, ugh, ph, or, w, ck>.
A grapheme is the smallest unit of writing that is distinctive for meaning, and is an abstract concept that can have varying physical realizations known as allographs.
For more detail on graphemes, read Terminology: Graphemes and Letters.