For those who never left the US, that's where you exchange your dollars for money :).
In all the years I spent in the US, I saw fewer exchange offices than I see here during a walk to downtown. That's what you get when you live on a continent with so many countries, even with the euro now being common to many of them.
With many of locals having cousins abroad, working throughout Europe as gastarbajter (gastarbeiter, guest worker, a German term which was adopted here in the sixties), there's a constant influx of currencies. Also, there's significant flow of tourists, foreigners of all designations (expats working for their patron companies, foreign students etc), so there's a lot of foreign cash around.
The currency laws being relaxed a lot now, anyone can open an exchange shop. You may need as little as 10.000€ to start a kiosk. The margins are ridiculously low - maybe 1 or 2% - but they make it up in throughput. The margin is lower when you're buying currency than when you're selling; sometimes you even get a discount if you're buying more. Most of the customers are selling 5, 10 or 20€ at a time, and they get the thick end of it. If you're buying 500€, you may pay just 0,2% above the official rate. Still incomparable with what it was in 2005, when the rate was fixed at 3%.
For the rare cases where they didn't put up a sign in English, the word is "menjačnica".
Paperwork is minimal, they just give you a receipt. No ID needed, nothing, unless you're selling US dollars, then they are required.
Update: no ID if you're buying or selling any other currency than the USD. The Americans spread their paranoia (and jurisdiction, shoving it to everybody else) over the world, so you may be ID'd and the banknote numbers typed into the computer. The data go to Narodna banka (people's, aka central bank) and then you may only guess where else - probably the Fed. So if you're looking to buy USD, it may take you several tries to find a place where they have them - nobody likes the paperwork, hence nobody likes the US dollar, hence it's scarce in retail.