Phoneme - more information
(This page is intended to supplement the simpler phoneme.)
The actual way that people produce phonemes varies. (In linguistics, we say 'the phoneme is realized differently by different speakers.') There is variation between dialects of English, as when the phoneme '-u-' as in 'but' sounds differently in the North of England, where it sounds rather like the '-u-' in RP 'put', and the South, where it is more like the '-u-' in RP 'cut'. Similarly, the phoneme '-a-' in 'bath' sounds quite like the (different) phoneme in RP 'bat' in the North, while in the South it is more like the exclamation 'Ah!'. There is also wide variation between individual speakers in the way that they produce the same phoneme - as is clear if we consider the phonemes '-r-' and '-s-'. Some individuals realise '-s-' as a sound very like '-th-' in 'think' (IPA: /θ/), a phenomenon due to the construction and/or articulation of their vocal organs. (There is a name for this usage, so common is it - it is a lisp.) Speakers of other languages who are learning English can have difficulties in the other direction - it is common among the British to parody a French accent by saying things like "Zeez ees ze sing I am sinking", for 'This is the thing I am thinking'.
The sound represented by '-r-' in writing is actually made in several different ways. Famously the Scots use a trilled '-r-' (the alveolar trill IPA: /r/) in a way that the English, who mostly use /r/, don't. In South Asia, it is often made in a retroflex style that is recognisably 'sub-continental' (/ɽ/ sometimes /ɺ̢/). French speakers commonly use the voiced uvular fricative or approximant /ʁ/ or the uvular trill (ʀ or /ɾ/) loosely called 'guttural'. In some native speakers' mouths, it sounds like 'w' - the late Roy Jenkins was known affectionately as 'Woy' because of his speech habit in this direction.
Whatever the precise phonetic distinctions that can be drawn between different individuals' realizations (pronunciations) of these sounds, there is a sense in which it is always the same unit in the language we are considering. So we say that '-r-', '-s-' and the rest are phonemes. (The individual variations are allophones.)
In other languages, of course, the phonemes vary. There is no difference in meaning in any pair of words in English that is marked by the difference between the usual sound of '-l-' and the 'dark' (velarized or pharyngealized) variant (/ɫ/) made by some speakers. (This is the sound that comes to resemble '-u-' in the cockney accent version of 'milk'.) However, in Welsh the two '-l-' sounds are separate phonemes: a word pronounced with a clear '-l-' may have a different meaning from one with the dark '-l-' sound, even if all the other phonemes in the word are identical. Scottish-accented English contains a phoneme now missing from RP - the consonant sound found in the exclamation 'och!', rather like the German for 'I' , 'ich'. This sound is phonemic in Scotland, where the words 'loch' (lake) and 'lock' are pronounced differently. In most of the rest of the English-speaking world, they sound identical. One marked difference between most American accents and most British accents is in the realisation of the sound of '-tt-' between vowels. In American accents, the word 'latter' often sounds exactly the same as a British pronunciation of 'ladder'. This is an allophonic variation, not a phonemic one.