So far, I have been able to live on non taxable income (scholarships, fellowships, etc). So I never had to learn much about how taxation works. But now that I am working “for real” (not that I dont consider PhD to be real work), I had to learn how the income tax (among others) work in my case.
As a Brazilian working in Japan as a professor, I actually have to option to pay either the brazilian or the Japanese income tax for two years. If I keep working in Japan for more than two years, then I have to pay Japanese I.T. anyway. It seems that this time limit varies with your job and nationality (probably based on international agreements).
So I since I have a choice, I had to research a bit. As for the tax amount, it is basically the same in both cases (around 20% of my salary). If I choose to pay taxes in Brazil, I need to send a notice to the japanese tax office about my decision, and pay my brazilian taxes directy to the brazilian government, yearly. On the other hand, if I decide to pay Japanese Income taxes, they will be deduced automagically from my salary, and I need to submit a form to the brazilian embassy stating that I have moved out of the country (along with a bunch of other info).
Since the values are about the same, I decided that it made more sense for me to pay the Japanese Income tax, since I should be staying here for the medium term. It was also generally less hassle. I was quite surprised about how much information about this is readily available online, though.