The futur simple expresses an action that will take place.  The futur antérieur expresses an action that will have taken place before another future action.  The future tense is used just like it is in English, however, in French, the future is always used after quand or lorsque (when), dès que or aussitôt que (as soon as) and tant que (as long as.)

To form the future tense, use the infinitive and add these endings that resemble those of avoir.  However, you drop the -e from -re verbs.

And of course, there has to be exceptions.  Here are the irregular stems for the future tense
(these will also be used in the conditional tense):

Other exceptions:  For appeler and jeter, double the consonant.  For nettoyer and payer, change the y to i.  For acheter, add an accent grave.  For préférer, the accents all remain the same.

To form the futur antérieur, use the future of either avoir or être (whichever the main verb takes) and the past participle of the main verb.

Quand ils reviendront, ils auront changé.  When they come back, they will have changed.
Dès qu'ils seront revenus, ils voudront repartir.  As soon as they have returned, they will want to leave again.