Since everyone else has given you a shitty explanation thus far, I'll give you a go:
Thelema is the practice of inversion.
Crowley did a lot of "magick". Essentially, he just ate feces and spoke whole paragraphs backwards. Supposedly, he would do absolutely as much as possible backwards, but the reasoning for these things wan't ever really something I could grasp.
He did the whole trying to summon shit, trying to possess or be possessed and all kinds of other weird shit.
Overall, they enjoy him because he more or less just tricked people. This illusory magick is what inspires [them] in many ways. For instance, they clamor for reparations on behalf of nogs by adorn them in "golden chains"… Go ahead and think about that situation for a minute.
A big thing that Crowley understood regarded symbology. Of this, we can find references or uses of sigils, which Grant Morrison conveniently explains (although you should be warned that these men are two ENTIRELY DIFFERENT BEASTS and where I find Crowley largely revolting, I actually appreciate Morrison) in a very eerie fashion.
The Seven-sided star was Crowley's invention. Its inversion probably means little to anyone ill-informed, as the number seven is highly important. See, five is the number of the human condition, six represents illumination or ascension (that's maybe an ambiguous word to use…) and seven is the holy number, divine incarnate. See, inverting the seven means obscuring divinity or casting a shadow of divinity. Now you get to learn about the Kabbalah and the Chakras. The Serpents cast a shadow and one can climb up the ladder or down the ladder the same (or so it appears).
This is already getting long so I'm going to stop there and let you ask questions if you're still curious.