What is the different between a pfo and ostium secundum?

Subtle: You're right that both can cause r to l shunt (and rarely l to r). But everyone is born with a foramen ovale (which is small) and when it fails to close, there is usually a flap and shunting only occurs with increased right atrial pressure (like during straining or cough). A secondum ASD is a fixed pathological defect in the interatrial septum. Size is variable. They're similar but different.

PFO vs ASD: Patent foramen ovale is communication between the right / left atria. It is common and a result of a failure for this communication to close when we are born. As a fetus this is needed because the babys lungs are not ready to oxygenate it bypasses the immature lungs of the baby and goes into moms circulation for oxygenating. It should close as babys lungs mature and take over. ASD is pathalogic.

Size: It's really just the size that distinguishes these. A pfo is small and rarely causes any problem. An ASD is larger and would only cause a problem if it is large enough to do so.