The letter S represents the voiceless alveolar sibilant /s/ in most languages and the IPA (International Phonetic Alphabet).
It also commonly represents the voiced alveolar fricative /z/, as in the Portuguese mesa or the English does. It may also represent the voiceless palato-alveolar fricative [ ʃ ], as in Portuguese, Hungarian, and German (before p, t). The letter S is the seventh most common letter in English and the third-most common consonant (after t and n).
In English, final ⟨s⟩ is the usual mark of plural nouns, and of third person present tense verbs.
(Or in other words)
She sells sea-shells on the sea-shore
What the caterpillar thinks of as the end of existence, the rest of the world calls a butterfly.