The letter s represents a voiceless dental or alveolar fricative in most languages. It's a common consonant and is the plural marker in English and other languages. However, the pronunciation of this consonant can vary.
In English this consonant is usually a voiceless alveolar fricative. This is not the case in the word his. Here it's a voiced alveolar fricative. In the word sugar, the s is a voiceless alveopalatal fricative and in treasure, the s is a voiced alveopalatal fricative.
The pronunciation of the letter s also varies in other languages. Between vowels, the s is /z/ in Italian. This is the case in the word casa (house). This intervocalic pronunciation is also used in French, Portuguese and German.
In Hungarian, the s is a voiceless alveopalatal fricative as in the English word sugar. This is exemplified in the word soha (never). The sound /s/ is spelled sz.
French has many silent letters, and the letter s is no exception. In the word vous (you), the s is silent. However, word-initial s is always pronounced /s/ as in sept (seven).
In many Spanish dialects, syllable-final s is glottalized. For example, the word costa (coast) is pronounced [kohta] in many dialects in addition to the pronunciation [kosta]. Besides these two pronunciations, costa can also be pronounced [ko:ta} in certain dialects. Here the s isn't pronounced at all, but as a result of compensatory lengthening, the vowel [o] becomes long.
The letter /s/ is a common letter of the Roman alphabet. Though usually pronounced as a dental or alveolar fricative, a number of other pronunciations are also possible. The pronunciation of the letter is most uniform in word-initial position and more varied when it's intervocalic and syllable-final.