Allophones are predictable phonetic variants of phonemes. Phonemes are units of sound which serve to distinguish one utterance from another. For example, the words "no" and "so" only differ by one unit of sound. This unit of sound can also be called a segment. In English /n/ and /s/ are phonemes. Many speakers of English are not aware of the great number of allophones in their language.
Compare the words "see" and "seed." The vowel sounds are not identical. The vowel sound of "see" is longer than that of "seed." Word-final vowels have a longer duration than those which come before word-final consonants. Compare "seed" and "seat." The vowel sounds of these two words are also different. The vowel sound of "seed" is longer than that of "seat." Vowels are longer before word-final voiced consonants than word-final voiceless ones. Thus, the longest vowel is in "see" followed by "seed" and then "seat." In fact, the vowel of "see" can be classified as long and the vowel of "seed" as half-long.
Compare the nasal of "my" with that of "emphasis." The two are not the same. In the word "my" both lips are involved in the articulation. This is a bilabial nasal. In the word "emphasis," however, a labiodental fricative follows. This is represented by the "ph" of "emphasis." In the production of "emphasis," the nasal is not bilabial. It is a labiodental nasal because the upper lip and teeth are involved in the articulation.
The vowels of "sad" and "sand" are not the same. In the former the vowel is oral. In the latter, however, the vowel is nasalized because it is followed by a nasal in the same syllable. In such environments, English has a rule which nasalizes the vowel. The two vowels are therefore allophones of one another. The nasalized vowel must occur before a tautosyllabic nasal.
Now compare the words "stone" and "tone." The alveolar plosives are not identical. In the word "stone," the alveolar plosive is unaspirated. In "tone," however, it is aspirated. This is entirely predictable. English has a rule which states that plosives are unaspirated when they are preceded by a syllable-initial alveolar fricative. Stated another way, plosives are aspirated when they are syllable-initial and followed by a stressed vowel. A liquid or glide may precede the stressed vowel as in "please," "pride" and "tune." It should be noted, however, that a number of English speakers do not pronounce "tune" with a glide.
All languages have allophones which are predictable phonetic variants of phonemes. Though all languages have many allophones, native speakers do not need to be taught to produce the allophones of their language. They do so automatically. For this reason, many are not familiar with the allophones of their language.