Phonemes are the units of sound (speech) which distinguish one word from another in a particular language. In most varieties (accents and dialects) of spoken English there are about 23 consonantal phonemes and 21 vowel phonemes (altogether 44 phonemes), but only 26 letters in writing. To see all the phonemes in English, the IPA, has a unique letter for every phoneme, unlike the English alphabet.
A good way to learn phonemes in a language is to learn minimal pairs. A minimal pair is a pair of different words that are different by only one sound. An example of this is "bit" and "bat". Even though only the vowel in each word is different, each word has a completely different meaning. Another example is "dip" and "tip", where both words have different meanings even though only the first consonant in each word is different.
- Concise Oxford English dictionary, p1027.