Grapheme different from phoneme
In linguistics, a grapheme is the smallest functional unit of a writing system.
The word grapheme is derived from Ancient Greek  γράφω (gráphō) 'write' and the suffix -eme by analogy with phoneme and other names of emic units. The study of graphemes is called graphemics. The concept of graphemes is abstract and similar to the notion in computing of a character. By comparison, a specific shape that represents any particular grapheme in a given typeface is called a glyph.Conceptualization
There are two main opposing grapheme concepts.In the so-called referential conception, graphemes are interpreted as the smallest units of writing that correspond with sounds (more accurately phonemes). In this concept, the sh in the written English word shake would be a grapheme because it represents the phoneme /ʃ/. This referential concept is linked to the dependency hypothesis that claims that writing merely depicts speech. 
By contrast, the analogical concept defines graphemes analogously to phonemes, i.e. via written minimal pairs such as shake vs. snake. In this example, h and n are graphemes because they distinguish two words. This analogical concept is associated with the autonomy hypothesis which holds that writing is a system in its own right and should be studied independently from speech. Both concepts have weaknesses.Some models adhere to both concepts simultaneously by including two individual units, which are given names such as graphemic grapheme for the grapheme according to the analogical conception (h in shake), and phonological-fit grapheme for the grapheme according to the referential concept (sh in shake).In newer concepts, in which the grapheme is interpreted semiotically as a dyadic linguistic sign, it is defined as a minimal unit of writing that is both lexically distinctive and corresponds with a linguistic unit (phoneme, syllable, or morpheme).Types of grapheme
The principal types of graphemes are logograms (more accurately termed morphograms), which represent words or morphemes (for example Chinese characters, the ampersand "&" representing the word and, Arabic numerals); syllabic characters, representing syllables (as in Japanese kana); and alphabetic letters, corresponding roughly to phonemes (see next section). For a full discussion of the different types, see Writing system § Functional classification.
There are additional graphemic components used in writing, such as punctuation marks, mathematical symbols, word dividers such as the space, and other typographic symbols. Ancient logographic scripts often used silent determinatives to disambiguate the meaning of a neighboring (non-silent) word.