Basically, "por" is used to express movement through time or space. This can be:

- A physical transition such as traveling

  • Viajé por Francia y España. (I traveled through France and Spain.)

- An exchange of objects

  • Pagaré $3 por este sandwich. (I will pay $3 in exchange for this sandwich.)

- A duration of time

  • Tengo que trabajar por ocho horas hoy. (I need to work for eight hours today.)

- A motivation/reason for doing something

  • Por su amor a los niños, quiere ser pediatra. (Because of her love for children, she wants to be a pediatrician.)

And "para" is generally used to convey destinations or end points. These can be:

- Physical locations

  • Salgo para Chile mañana. (I leave for Chile tomorrow.)

- People (recipients)

  • Este regalo es para Adela. (This gift is for Adela.)

- Deadlines

  • Necesito la presentation para el viernes. (I need the presentation by/on Friday.)

- Goals

  • Trabaja mucho para ganar más dinero. (He works a lot in order to earn more money.)

There are also many expressions that are used with por and para. Some of the most common are:

para siempre forever por ejemplo for example
para variar just for a change por favor please
para que so that/in order that por ciento percent
    por supuesto of course
    ¿por qué? why?
    por fin finally