* As a preposition (meaning "at the back of"): derrière
* As an adverb (meaning "in the past"): derrière or en arrière
Here are some examples:
* The house is behind the trees. La maison est derrière les arbres.
* We are behind schedule. Nous sommes en arrière sur le planning.
* Don't look behind you! Ne regarde pas derrière toi!