Proper use of the preposition por can be difficult for non-native Spanish speakers because of the variety of meanings. Por may be used...

to express the reason or motive for an action
Lo hice por tí.
I did it for you.

Cancelaron el partido por la lluvia.
They canceled the game because of the rain.

to indicate a place where movement is occurring
Los ladrones entraron por la ventana.
The burglars came in through the window.

Vamos por la calle sexta.
We're going by way of Sixth Street.

to indicate periods of time: parts of the day
por la mañana - in the morning
por la tarde - in the afternoon
por la noche - in the evening
por la noche - at night
por la madrugada - in the early morning

to mean per
Íbamos a 80 millas por hora.
We were going 80 miles per hour.

Gano diez dólares por hora.
I make ten dollars per hour. 

to express price or an exchange
Te doy mi reloj por tu bicicleta.
I'll give you my watch for your bike.

Lo compré por quince dólares.
I bought it for fifteen dollars.

to use in the formula
por + means of transportation
Vine por avión.
I came by plane.

Fuimos a Veracruz por autobús.
We went to Veracruz by bus.

to make statements using the passive voice (por = by)
Esa canción fue escrita por Tracy Chapman.
That song was written by Tracy Chapman.

the object of an errand with verbs like: ir, enviar, mandar, venir, etc.

with estar to express "to be about to"