που: It means which/who/that. It is used in relative clauses.
Η ταινία που είδα χθες ήταν αστεία. = The movie (that/which) I watched yesterday was funny.
Ο άντρας που είδαμε ήταν ο πατέρας μου. = The man (that/who) we saw was my father.
It can be used about things or persons.
πως: It means ''that''. It is used in relative clauses.
Examples: Έμαθα πως έλεγε ψέμματα. = I found out (that) he was lying.
Μου είπε πως θα με πάρει τηλέφωνο. = He told me (that) he would call me.
Άκουσα πως παντρεύτηκες. = I heard (that) you got married.
It doesn't refer to things. You use it when you want to state something that you learned, heard from someone. ''πως'' can be replaced by ''ότι'' in these cases.
Just for better comparison and understanding about ''που'' & ''πως'', I am also writing about the cases of ''πού'' & ''πώς''.
Πού: It means ''where''. You use it when you ask a question.
For example (as stated above): Πού είσαι; = Where are you?
You should also use it in indirect speech.
For example: Με ρώτησε πού ήμουν. = He asked me where I was.
Πώς: It means "how". It is also used when questioning (as shown by Ioannis) and in indirect speech.
Με ρώτησε πώς ήμουν. = He asked me how I was.
I hope that was helpful :)