How do sites get my city?

[Yeti]

You reveal your IP address to any website you visit, and since they're assigned in blocks to various ISP's, etc., they can usually pinpoint you to at least your city. Some browsers also have a geolocation switch you may need to set to "off" to not reveal more precise information.

And if you feel uncomfortable with that, you should probably be aware that when Google has driven around photographing streets for Google Maps, they've also picked up data on which wireless networks are functioning in any given area (called Google Location Services). Now, if you use a cell phone with Google services on it (like Google Maps, as most people have), they can rapidly know where you are within something like 50 feet just by recognizing which wireless networks are in the area around you (operating out of homes, businesses, etc.). And when you realize that operations like Google and Facebook are also developing massive network "maps" of who's connected to whom, it can get a little scary just how much they're able to monitor about you, and how much they could even "infiltrate" your social network if they wanted to influence you, or plant a product for you to buy, etc.

Anyway, there aren't any major known threats from revealing your IP address and related info (beyond someone being able to rapidly make up ads that appear directed at you), but you should probably be aware that whatever site you visit can get a decent amount of info like that from you (including what computer browser and operating system you're using, what your display dimensions are, and so on). And I'd seriously be more concerned about what exactly you're potentially revealing to whom in the big scheme of things if you start putting up personal information any place like social networks, even if you think it's "private."