It is easier to ask this question than to answer it: we cannot simply give a single description for each phoneme, because, in most cases, the pronunciation of a phoneme may vary according to where it occurs in the word (initially or finally) or what other phonemes it is adjacent to. In general, we say that the pronunciation of a phoneme depends on the phonological context. Take the vowel of Dutch biet as an example. Suppose you were to teach a foreign learner of Dutch how to pronounce this vowel, and suppose that the learner mastered the pronunciation of that vowel, and used it in bier. He would, of course, mispronounce the word, because in the phonological context ‘before /r/ in the same word’ the vowel of Piet is pronounced long. (The same goes for the vowels in koe and nu: please check this for yourself.) A vowel like GA /æ/ is pronounced longer in cab than in cap. Or again, GA /t/ in style is pronounced very much like AN /t/ in stijl, but in tile it is pronounced with aspiration, not unlike the [t] that may be heard from Groningen speakers of Dutch, as in Martinitoren. The different pronunciation of a phoneme that is used in different phonological contexts is known as an allophone. Fortunately, there is a lot of sense and system in how the various allophones of a phoneme are pronounced and when they are used. With respect to the aspiration of GA /t/, for instance, we need only remember that when they occur at the beginning of a syllable, GA /p, t, k/ are aspirated, as in par, tar, car, but not when /s/ precedes in that syllable, as in spar, star and scar.
Summarizing: a language has vowels and consonants, called phonemes. The different pronunciations of a phoneme in different phonological contexts are called allophones. It is possible to give rules saying which allophone or allophones of a phoneme, or group of phonemes, occur in which phonological contexts. Following long-standing practice, phonemes are written between slashes: / /, and allophones between square brackets: [ ], as for instance in ‘GA /t/ is pronounced [th] at the beginning of a syllable, but [t] when /s/ precedes.’