After completing some challenges, I've noticed I have some unspent Badass tokens on the Badass Rank tab. The description text says:

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> Complete challenges to earn Customization items and Badass Ranks. Badass ranks earn you tokens which can be spent on permanent character upgrades that apply to ALL characters associated with your gamer profile.
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How do these ranks and tokens work? I'm Badass Rank 9, which gave me three tokens. When I spend these tokens, are the effects I can unlock random, or is there a set progression somehow? What are the Customization items?

In general, how do Badass Ranks, tokens, and Customization items work?