What is a Phonetic Script?

The individual sounds of human languages are called phonemes in linguistics. A phoneme can be defined as 'a distinct unit of sound' or 'the smallest unit of speech distinguishing one word (or word element) from another'. Think of the 'd' sound in dog. It is not 'tog' or 'bog' because they begin with different phonemes. In linguistics phonemes can be annotated /d/ as compared to /t/ or /b/. Although one might simply say that these examples begin with a different letter, it is not true that all phonemes are interchangeable with letters.
For one thing, the IPA uses a single symbol to represent what in English, for example, may require two 'letters' ('th', 'sh', etc). Also, no single language uses all the different sounds that are used in speech worldwide. Furthermore, not all languages use the same alphabet 2 , any alphabet, or indeed, are written down at all. The IPA encompasses all these sounds and includes additional specific notation marks, called diacritics, to represent fine distinctions in consonant sounds and to show nasalisation, length, stress, and tones of vowels. A single 'phoneme' may have one or more variants in the way that it is articulated, depending on its position in a word within an individual language, for instance. Diacritical marks enable linguists to show these distinctions.
KAYNAK:http://www.ibu.edu.tr/akademik/fyaman/