Human speech is produced by a restriction — and then release— of air somewhere within the breath channel (mouth, nose, throat). (When air passes through the breath channel without restriction, we call that simply breathing.) Not all sounds made by a restriction and release of air are speech sounds — snoring, sneezing, and slurping don't qualify.
The range of sounds that are produced by the restriction and release of air and which can be distinguished as speech sounds by native speakers of a language roughly form the sounds from which a language's alphabet is formed, though there are usually more speech sounds than there are letters of the alphabet. Most alphabets have about 25 sounds along that range for which distinct written characters are provided. English has 26 letters; Greek has 24.
These sounds that make up the letters of the alphabet are divided into two main groups: vowels and consonants. The sound of both vowels and consonants is produced by a restriction of air. The restriction that produces vowels is at the weak end of the range while something called hard consonants are produced at the strong end of the range.
Try pronouncing the letter 'A' [a vowel] and the letter 'K' [a hard consonant].
That will help demonstrate the range of restriction of air involved in the
production of human speech.)
In English, there is a long 'O' sound (pole) and a short 'O' sound (pot), both represented by the one letter ').' Greek chooses in this case to represent the two sounds by distinct letters: for long 'O' (o-mega), and for short 'O' (o-micron).
Some alphabets have what seems to be unnecessary letters. English, for example, has the letters 'C' and 'K.' Greek uses only 'K' (kappa).
In some cases, a sound that is represented by a distinctive letter in one language is represented by a combination of letters in another language. For example, Greek has a distinctive letter (theta) for the sound represented in English by a combination of the letters 'T' and 'H.'
Sometimes a sound will be represented in a language both by a distinctive letter and a combination of letters, as with the 'F' sound in English, which can be represented either by the letter 'F' or by the combination of the letters 'P' and 'H' (ph). Greek uses only one letter (phi).