What is a "gaijin-card"?

I was reading a forum somewhere where foreigners were asked for their "gaijin-card." They were stopped randomly by police and asked to show a card, but they never explained on the forum what it was exactly. How do you get it? How is it used exactly? Do I really need it if I decide to travel to Japan?

Thanks

"Gaijin card" is of course not the official name - it is "Alien Registration Card," so you might see the abbreviation ARC here and there as well.

This does not apply if you are just coming as a tourist.

If you come to Japan and stay/live for 90 days or longer, and hold a fixed address, you will need to register with the local city hall as a resident. And you get this card issued as an ID. On it is information like name, gender, date of birth, address, visa status & validity date, occupation/employer, etc., and as a resident if you move within Japan or change occupation, you report the change to the city hall and it gets updated.

In Japanese language, this card is called "gai-koku-jin-toroku-shomeisho", so in short it is called "gai-jin card" among expats. It proves you're not an illegal resident. If you are a tourist and asked to show your ID, you can just show them your passport instead.

Thank you so much for answering.