Most of the time, the s of Spanish sounds the same as the "s" sound in English words such as "see" and "bus," although perhaps a bit shorter. However, the sound of the Spanish s is also affected by the sound of the letter that follows it. When an s is followed by a voiced consonant in other words, a b, d, voiced g, m, n, l, r or v it is pronounced like a soft "z" sound.
Note that the "z"-like sound occurs in Spanish only before those consonants. It does not occur at the end of words (such as in plurals) or when followed by a vowel. The s sound changes slightly merely because it is blending into the sound the follows.
In some areas, native speakers frequently omit the s sound when it comes at the end of a syllable, so that "¿Cómo está usted?" ends up sounding something like "¿Cómo etá uted?" You should be aware of this if you're traveling in such areas but shouldn't imitate it elsewhere.
The question "¿Cómo está usted?" (How are you?) is used in our audio lesson on pronouncing the Spanish s. Words used to demonstrate the "z"-like sound of s are mismo (same) and desde (from).