The Spanish (and Catalan and Basque) S is an apicoalveolar fricative (with the end of the tongue on the alveols); the English S (and Basque Z) is a laminodental fricative (the sound is produce with the blade of the tongue).
http://en.wikipedia.org/wiki/Voiceless_alveolar_fricative
It is one of the erroneous pronunciations that identify most of Spanish speakers when speaking English and viceversa (the other one is the Spanish T and D versus the English T and D).
Because of the influence of some languages in American Spanish, in some of the Spanish dialects in
America the S can be pronounced as the English S, but that is not the standard (and original) pronunciation.
Lumia
http://onomastica.mailcatala.com