i've read about CDN but dont know exactly what it is for.

lets say i've got an international social network (text and image content), and it's growing in traffic from different countries, do i have use of CDN?

the picture i got from the sources i've read is that it copy your content and put it in many servers spread out over the world so that users will fetch it from the nearest point.

does this mean that every server has a copy of my mysql database and the image files?

is this the proper way to make your web service available for the world? cause how else could you set up a servers through out the world, contacting hosting companies for each country?