They are different types of pronouns. Je is a personal pronoun; and can be used both as subject pronoun as object pronoun. If a subject pronoun is used [eg: the you in 'you are trustworthy'] you can either use Je or Jij [je/jij bent betrouwbaar]. They are completely equivalent. If an object pronoun is used [eg the you in 'I trust you'], you can either use Je or Jou [Ik vertrouw je/jou]. They are completely equivalent as well. However do note: jij is only a subject pronoun, and jou is only an object pronoun.
Now there's a different word that looks very much alike: Jouw. Jouw is a possessive pronoun. It's your cat - so: Het is jouw kat.
So for instance correct grammatical use of these words might be: Jij stelde jouw vraag; ik antwoordde jou. [You asked your question, I answered you].