What is the difference between "por" and "para"?

"Por" and "Para" are two words that both mean for, but are not interchangeable; their usage depends on the context of a sentence. "Por" is used when referring to to cause, travel and duration, while "para" is used when referring to effect, destination and deadlines.
For example "Voy a comprar esto libro por mi madre" would refer to cause as you are buying the book because your mum asked you to, but "Voy a comprar esto libro para mi madre" would refer to effect as you are buying the book as a present for your mum. "Vamos por Alemania" would refer to travel, as it would mean "Let's go through Germany" and "Vamos para Alemania" would refer to a destination, meaning "Let's go to Germany". Finally, "Hacemos nuestra tarea por dos horas" refers to duration, meaning "Let's do this task for two hours" while "Hacemos para las seis" refers to a deadline, meaning "Let's do this task by 6'o clock".