As mentioned, AdSense uses geotargeting to display ads to your users. This means ads are targeted by IP address to the location of your users.
So, if I look at your site from here in Mountain View, California, I'll see ads targeted to U.S. users. If your users in Singapore look at the same page, they'll see ads targeted to a Singaporean audience.
AdWords advertisers can choose which countries they'd like to target with their ads. Therefore, you may see U.S. advertisers targeting ads to your users in Singapore.
If you'd like to see which ads your users in other countries are seeing, you can download the Google AdSense preview tool which will allow you to do this:
Is that a way to see what ads appear for which countries? I mean, if I am creating a website targeting UK visitors, I do like to know what ads they are seeing on my site and I am certainly not planing to flight over there to check out.