When do you use the prepositions 'por' and 'para'?

Prepositions are used to relate a word or element in a sentence. 'para' and 'por' are the translation of 'for'. 'Para' is used when there is a destination eg. 'para ti', 'para mi', 'para nosotros'. (for you, for me, for us). However 'por' would be used in the context of doing something FOR someone eg. 'arriesgo mi vida por ti' (I am risking my life FOR you).