Por and para both mean for.
However, they are not interchangeable.

     Use para to indicate purpose or use :
Sonia va al supermercado para comprar el pan.
       Sonia goes to the grocery store in order (for)
        to buy the bread.

     Use para to express destination: recipient (a person):
María va a comprar un regalo para él.
       Mary is going to buy a gift for him.

     Use para to express destination: location (a place):
¿A qué hora parte un tren para Berlín?
       What time does a train leave for Berlin?

     Use por to indicate reason or motive:
Estoy aquí por asuntos de negocios.
       I am here because of (for) business reasons.

     Use por when discussing time or day:
Hago la tarea por las noches.
       I do homework at night (for nights).

     Use por when expressing exchange:
Te doy mis palabras por tu dinero.
       I give you my words for your money.