What's the difference between 'por' and 'para'?

'Por' and 'para' both usually translate into English as 'for'. There is no firm distinction between the two so it is necessary to learn when you would use either 'por' or 'para'.
Por:Duration of time e.g. Trabajo por cinco horas (I work for 5 hours)Periods of time in the day e.g. por la tarde (for/in the afternoon)Direction e.g. el coche va por la calle a la izquierda (the car goes down the street on the left)Number phrases e.g. veinte por ciento (twenty percent)Exchange e.g. Pagó dos euros por el libro (He/she paid two euros for the book)On behalf of e.g. Lo hace para ti (I'm doing it for you)After gracias e.g. Gracias por la comida (thank you for the food)Para:Destined for e.g. Tenemos suficiente comida para todos (we have enough food for everyone)Purpose of something/reason for doing something e.g. La medicina es para curarte (the medicine is for curing you)Deadline/time limit e.g. Tenemos que salir para la una (we need to leave by one)Time period e.g. Quiero el coche para tres días (I want the car for three days)Comparison e.g. Ella es alta para su edad (She's tall for her age)Opinion e.g. para mí (for/to me)Idiomatic expressions e.g. está para llover (it's going to rain)