Sorry I don't have time to put together all the sources involved, but in general the basic confusion will come from the fact that there are two general views on the nature of Jewish eschatology, and you can read someone saying something about it and not quite know which model they are working with.
One is generally attributed to the Rambam. Call it the philosophical model. The other is generally attributed the Ramban, call it the mystical model.
Following in chronological order:
In both models, after someone passes away, their soul has the potential to go to a World of Souls. In the mystical model this is "Gan Eden", and in either one this is one potential meaning of Olam HaBa. The primary focus of it, in both models, is that the soul has an increased understanding and awareness of G-d, which is a great reward (compare to the pleasure in discovering a new idea or truly understanding something you were struggling with, but many times more). Gehennom is a place where souls are purified from negative effects of sin and other aspects of the world in order to be able to have that experience.
In both models, there is a messianic era. How "natural" this era will be is somewhat a point of divergence. The main focus of this era is the total unencumbered proper observance of G-d's commandments. This is not generally called "Olam HaBa" except when the term is being used as a vague generality.
In both models, there is a Resurrection of the Dead. This is the other potential meaning of Olam HaBa and it is a time of living in this physical world, but in a more miraculous state. It is the understanding of the purpose of this world where the models diverge.
The philosophical model views this world (call it World of Resurrection) as a "temporary" stage where those who are there further prepare themselves for the true end - the World of Souls. Under this model, the Rambam suggest Gan Eden is actually a physical place of great comfort in the messianic era, although he seems tentative about it. Does everyone die in order to be resurrected? Not necessarily.
In the mystical model, however, the ultimate end is the World of Resurrection. The World of Souls is preparatory to it. Certainly according to the Zohar, everyone needs to die and be resurrected for the proper experience of that state.
To the degree that there are no Halachic implications to either of the models, there is no determination in Judaism that one is correct and the other isn't. We will get to find out ;-).
Non-Jews have a portion in the Olam HaBa (meaning the final one) by following the Nochaide Laws.
Now in all the above you can find plenty of discussion and divergence in detail, but hopefully this will give a guide to help orient someone in the discussion.