Nessuno / nessuna can be adjective and pronoun. Therefore, it can't be translated always with nobody, because nobody can be used only as a pronoun. As an adjective the translation to English is just no (nessun uomo = no man).
example
Nessuno lo sapeva.
Nobody knew it.
Concerning following vowels the same rules applies as in the case of the indefinite articles. The apostroph is only used in relationship to feminine nouns (nessun' amica but nessun uomo).
