Phoneme and letter represent respectively the sounds (speaking) and the graphic signs (writing). Phonemes are the sound units that make up the speech and letters, are the graphic signs that make writing possible. Together and in an a certain sequence and form, letters make up the alphabet.

Example 1
coçar = 5 letras
/k/ /o/ /s/ /a/ /r/ = 5 fonemas
Example 2
máximo = 6 letras
/m/ /á/ /s/ /i/ /m/ /o/ = 6 fonemas
Although the number of phoenems and letters coincide in many words, it is not always the case.
Exemplo 1:
acesso = 6 letras
/a/ /c/ /e/ /s/ /o/ = 5 fonemas
Exemplo 2:
chute = 5 letras
/x/ /u/ /t/ /e/ = 4 fonemas
How are Phoenems classified?
We can classify these by vowels, consonants and semi-vowels ( vogais, consoantes e semivogais).
Vogais / Vowels
Are all sounds issued without obstacles only throuth the mounth (a, e, i , o, u), or by the mouth and the nasal cavities (ã, ẽ, ĩ, õ, ũ).
Examples:
pia, ando, cesto, quero, lente, li, lindo, sonho, avó, som, susto, untar.