"Shall" is a rather tricky word since it lacks a concrete definition. It's what you would call an auxiliary verb--a verb that doesn't necessarily mean anything by itself, but which serves a purely grammatical function. Take this simple example:
Does he go? (Est-ce qu'il va ?)
Did he go? (Est-ce qu'il est allé ?)
Would he go? (Irait-il ?)
Will he go? (Ira-t-il ?)
Shall I go? (Préférez-vous que je m'en aille ?)