Phonemes are the smallest units of sound that distinguish one word from another in a language. There are 44 phonemes in English represented by letters alone or letter combinations. Phonemes can be segmented from words by breaking words into their individual sound units. The English language has a complex phonetic code where one phoneme can be represented by 1 to 4 letters. Minimal pairs are word pairs that differ by one phoneme, like "fan" and "van", helping learners distinguish similar sounds.