With regard to the difference between a Gilgul (where a Nefesh is reincarnated during the time of birth) to an Ibur (where a Nefesh is enclothed in a body of a person during his life) -- there are two different types of Gilgul, and there are two different types of Ibur, and these are: 1) a gilgul where ONE Nefesh enters into the body of an Adam when he is born and he comes out into the air of the world, and 2) sometimes it is possible that TWO Nefashot will reincarnate together, also when the Adam is born; this is called a "Gilgul Kaful" ("Double Gilgul"), as we explained before (and as we will explain later).
Here, both of their Nefashot reincarnate together, and they come into the world at the time the body of the Adam is born, and they [the two Nefashot] are not separated at all until the day of their death. [Watch the tenses in the Hebrew here; the uses of plural and singular forms is interesting.] Both of them are called "one Nefesh" (and they are connected as one), and they are formed as one Nefesh.
And, they both suffer the hardships that come to the body all of their life, and they both experience the pain of death. However, with an Ibur, there is not entry of an Adam at the time of his birth, just as we described above. There are two types [of Ibur]: 1) When a Tzaddik comes for his own benefit, he enters (he 'iburs') into an Adam to complete whatever thing it is missing (for example, if he did not have the chance to do a particular mitzvah in his life), just as it will be explained in its place [in the Hakdama 11]. [The second type of Ibur is] 2) when he comes for the benefit of the Adam, to help and raise him in his (observance of) Torah and mitzvos. [He still benefits from them, but the focus is on the benefit of the other.]
When he comes for his own benefit (Ibur Type 1), he does not enter the Adam until he is 13 years old and one day, because only then the child is obligated to Torah and mitzvos. Once he is this age, he [the Adam] can rectify not only himself [his own Nefesh], but also the Neshama of the Ibur. Thus he does not enter before this time (of Bar Mitzvah), only afterwards, when he is obligated to do mitzvot.
Then, he will enter and spread into the body, similar to the way that the Nefesh of the Adam himself spreads in the body, and both of the can suffer and experience the hardships that come to this body, and they suffer together, and the ibur remains there for whatever time it was set [by the heavenly court] for it to be there to rectify and complete whatever it needed to, and [when complete,] it can leave while the body is still alive, and it can return to its place above in Gan Eden.
However, when it comes for the benefit of the Adam and not for his own benefit (Ibur Type 2), he comes according to his own volition [not according to the decree of a heavenly court], and not based on a directive. Thus he is not obligated to experience the pain of the body at all, and it does not feel at all the hardships and difficulties that come to the body. And if it is no longer comfortable for it to remain with the body, he can leave it at will, as it says, "סורו נא מעל אהלי האנשים הרשעים האלה וגו'."