In contrast, a phoneme is a speech sound in a given language that, if swapped with another phoneme, could change one word to another. Phones are absolute and are not specific to any language, but phonemes can be discussed only in reference to specific languages.
For example, the English words kid and kit end with two distinct phonemes, /d/ and /t/, and swapping one for the other would change one word into a different word. However, the difference between the /p/ sounds in pun ([p?], with aspiration) and spun ([p], without aspiration) never affects the meaning or identity of a word in English; it's not possible to replace [p] with [p?] (or vice versa) and thereby convert one word to another. Thus, [p?] and [p] are two distinct phones but not distinct phonemes in English.
As can be seen in those examples, phonemes, rather than phones, are the features of speech that are typically reflected (more or less imperfectly) in a writing system.
In the context of spoken languages, a phone is an unanalyzed sound of a language (Loos 1997). A phone is a speech segment that possesses distinct physical or perceptual properties and serves as the basic unit of phonetic speech analysis. Phones are generally either vowels or consonants.
A phonetic transcription (based on phones) is enclosed within square brackets ([ ]) rather than the slashes (/ /) of a phonemic transcription (based on phonemes). Phones (and often phonemes also) are commonly represented by using symbols of the International Phonetic Alphabet (IPA).
For example, the English word spin consists of four phones, [s], [p], [?] and [n], and the word thus has the phonetic representation [sp?n]. The word pin has three phones; in that word, the initial sound is aspirated and so can be represented as [p?]; the word's phonetic representation would then be [pn]. (The precise features that are shown in a phonetic representation depend on whether a narrow or broad transcription is being used and the features that the writer wishes to draw attention in a particular context.)
When phones are considered to be realizations of the same phoneme, they are called allophones of that phoneme (more information on the methods of making such assignments can be found under phoneme). In English, for example, [p] and [p?] are considered allophones of a single phoneme, which is written /p/. The phonemic transcriptions of those two words is thus /sp?n/ and /p?n/, and aspiration no longer being shown since it is not distinctive.