My professor in college who taught a course in "Indian Philosophy" reminded us that one of the most important things to know about Buddhism is --
1) what many Buddhists consider to be the fundamental core teaching of the Buddha:
"All is Dukha."
And what is Dukha...? The best translation, according to my professor (who knew the requisite languages) is "frustration".
Thus, for the Buddha, the first step toward "enlightenment" is to realize that everything is frustration, confusion, darkness.
From there, he taught a "six-fold path", which includes the renunciation of everything, in order to cut all ties to the welter of existence that imprisons us in Dukha.
Part of this "enlightenment" is the realization that the prison of existence is an illusion of the mind. And part of the welter of illusions that imprison the mind is the very notion that there is a "self" in need of escaping the prison. Some of the more radical sects of Buddhism (like certain types of Japanese Zen) go so far as to say that God is also an illusion, along with Self and World.
2. The second most important thing about Buddhism, according to my prof., is that it is not a unified simple belief system: just like any other belief system, there is plenty of diversity and competing, even conflicting, schools of thought and sects and splits, etc.
In this panoply of diversity that is Buddhism, one could say that Tibetan Buddhism is the most "religious" in that it really indulges in all the religious symbolism and claptrap typical of religions (spirits, angels, demons, hells, and so many complex rituals it makes Catholicism seem like Unitarianism) -- while Zen Buddhism, on the other hand, tries to be so radical and simplex (in fidelity to the original teachings of Buddha, supposedly), it could almost be said to be "against religion".