Futur simple
The futur simple is made by simply taking the infinitive of the verb and adding the correct form of avoir (to have) to the end of the word. In the nous and vous form of the word, the ending is instead just -ons and -ez, respectively. However, there are also some French verbs for which an irregular stem is used, such as aller (to go, futur simple stem = ir) and etre (to be, futur simple stem = ser). For instance:
Je mange I eat
Je mangerai I will eat
Nous allons We go
Nous irons We will go
The futur simple usually refers to events that will happen further away in time than the futur simple.
Futur proche
The futur proche uses the correct present form of aller (to go) and then has the infinitive after: je mange, je vais manger = I eat, I will eat.
Notice that the futur proche, which resembles the be-going to future, actually translates as the will future, while the futur simple is the opposite!
See also: past tense, present tense, grammatical aspect.