le futur proche v. le futur simple

French has two future tenses -- the futur proche and the futur simple. The futur proche is formed with the auxiliary aller which is followed by an infinitive (Je vais partir. 'I'm going to leave'). The futur simple doesn't have an auxiliary. Instead, the infinitive form becomes the stem to which future endings are added (Je partirai .  'I will  leave').  For more details  on the formation of these two future tenses, including  irregular  forms, see the related links:  future: regular,  future: irregular, and futur proche.

futur  proche vs.  futur simple

The two tenses are virtually interchangeable in most contexts, especially in spoken French. 

So what is the difference? In general, the two  tenses differ  in their level of formality; the futur proche is used  in more informal contexts and the futur simple  in more formal contexts. Thus, the futur proche is primarily used in speech and less frequently in writing. The two tenses also differ in their relative distance to the present moment. The future proche, also called the futur immédiat, typically refers to a time very close to the present moment, i.e, the near or immediate future. The futur simple, on the other hand, is often used for events in the more distant future. Because the futur simple is associated with distant future events, it often takes on a detached, objective  quality making it the preferred tense for future events that represent general truths.

The two tenses also indicate a difference in the speaker's perception of the future event. The futur proche indicates that the speaker is relatively certain that the future event will actually happen. In contrast, the futur simple indicates that the speaker is less certain of the future event happening. Let's imagine a context to make this distinction more clear.

Suppose that a baby is learning to walk. Suddenly, he totally loses his balance. His mom says:

In this scenario, the first sentence, the one in the near future, sounds more appropriate in French and in English. Why? Because when someone completely loses his balance, he is going to fall. So the futur proche indicates both the immediacy of the future event and its inevitability in the mind of the speaker. 

Of course, speakers are usually more certain about the immediate future and less certain about the distant future. Does this mean that certainty and distance to the present always go together? Not exactly. One can be certain about events in the distant future too. For example: Je vais avoir un enfant: the speaker is pregnant.  J'aurai un enfant: the speaker is thinking about the future and may not be pregnant.

differences between French and English

In general, the future tenses in French and in English are used similarly. However, there is one major difference in future tense usage between the two languages. French requires the future tense after certain conjunctions (dès que, aussitôt que [=as soon as], lorsque, and quand [=when]) where English usage calls for the present tense.

In the French examples above, the subordinate clauses starting with dès que,  aussitôt que,  lorsque,  and quand are  in the simple future because the main clauses are in the future. The action in the subordinate clause implies future tense since it will take place at around the same time as the action in the main clause. (Note the use of the present tense in the English translations).