In Spanish, the letter s is pronounced exactly as in English. The letter z is pronounced the same, as an "s" in Latin America, however in Spain it is pronounced similar to "th" as in the word "thin".
Note: The examples below are with the Latin American pronunciation.

s
Esta es mi esposa.
     This is my wife.
la semana pasada
     last week
unas sillas.
     some chairs.
Son niños traviesos.
     They're mischievous children.
Son niños sumisos.
     They're submissive children.
z
El zapatero está triste.
     The shoemaker is sad.
Mi collar tiene zafiros.
     My necklace has saphires.
El zopilote es negro.
     The turkey volture is black.
Pelota azul
     Blue ball.
té con leche y azúcar.
     Tea with milk and sugar.