A writing system comprises a set of symbols, called a script, as well as the rules by which the script represents a particular language. The earliest writing appeared during the late 4th millennium BC. Throughout history, each independently invented writing system gradually emerged from a system of proto-writing, where a small number of ideographs were used in a manner incapable of fully encoding language, and thus lacking the ability to express a broad range of ideas.

Writing systems are generally classified according to how their symbols, called graphemes, relate to units of language. Phonetic writing systems – which include alphabets and syllabaries – use graphemes that correspond to sounds in the corresponding spoken language. Alphabets use graphemes called letters that generally correspond to spoken phonemes. They are typically divided into three sub-types: Pure alphabets use letters to represent both consonant and vowel sounds, abjads generally only use letters representing consonant sounds, and abugidas use letters representing consonant–vowel pairs. Syllabaries use graphemes called syllabograms that represent entire syllables or moras. By contrast, logographic (or morphographic) writing systems use graphemes that represent the units of meaning in a language, such as its words or morphemes. Alphabets typically use fewer than 100 distinct symbols, while syllabaries and logographies may use hundreds or thousands respectively.

According to most contemporary definitions, writing is a visual and tactile notation representing language. As such, the use of writing by a community presupposes an analysis of the structure of language at some level. The symbols used in writing correspond systematically to functional units of either a spoken or signed language. This definition excludes a broader class of symbolic markings, such as drawings and maps. A text is any instance of written material, including transcriptions of spoken material. The act of composing and recording a text is referred to as writing, and the act of viewing and interpreting the text as reading.

The relationship between writing and language more broadly has been the subject of philosophical analysis as early as Aristotle (384–322 BC). While the use of language is universal across human societies, writing is not; writing emerged much more recently, and was independently invented in only a handful of locations throughout history. While most spoken languages have not been written, all written languages have been predicated on an existing spoken language. When those with signed languages as their first language read writing associated with a spoken language, this functions as literacy in a second, acquired language. A single language (e.g. Hindustani) can be written using multiple writing systems, and a writing system can also represent multiple languages. For example, Chinese characters have been used to write multiple languages throughout the Sinosphere – including the Vietnamese language from at least the 13th century, until their replacement with the Latin-based Vietnamese alphabet in the 20th century. Likewise, closely related variants of the Latin alphabet have been used to write most European languages, entirely irrespective of the degree of mutual intelligibility (or lack thereof) between them.

In the first several decades of modern linguistics as a scientific discipline, linguists often characterized writing as merely the technology used to record speech – which was treated as being of paramount importance, for what was seen as the unique potential for its study to further the understanding of human cognition.

While researchers of writing systems generally use some of the same core terminology, precise definitions and interpretations can vary by author, often depending on their theoretical approach.

A grapheme is the basic functional unit of a writing system. Graphemes are generally defined as minimally significant elements that, when taken together, comprise the set of symbols from which texts may be constructed. All writing systems require a set of defined graphemes, collectively called a script. The concept of the grapheme is similar to that of the phoneme in the study of spoken languages. Likewise, as many sonically distinct phones may function as the same phoneme depending on the speaker, dialect, and context, many visually distinct glyphs (or graphs) may be identified as the same grapheme. These variant glyphs are known as the allographs of a grapheme: For example, the lowercase letter ⟨a⟩ may be represented by the double-storey |a| and single-storey |ɑ| shapes, or others written in cursive, block, or printed styles. The choice of a particular allograph may be influenced by the medium used, the writing instrument used, the stylistic choice of the writer, the preceding and succeeding graphemes in the text, the time available for writing, the intended audience, and the largely unconscious features of an individual's handwriting.

Orthography (lit. 'correct writing') refers to the rules and conventions for writing shared by a community, including the ordering of and relationship between graphemes. Particularly for alphabets, orthography includes the concept of spelling. For example, English orthography includes uppercase and lowercase forms for 26 letters of the Latin alphabet (with these graphemes corresponding to various phonemes), punctuation marks (mostly non-phonemic), and other symbols, such as numerals. Writing systems may be regarded as complete if they are able to represent all that may be expressed in the spoken language, while a partial writing system cannot represent the spoken language in its entirety.