From the Antiquity people used natural sources of the relief to separate their countries. These we are calling physical boundaries. For instance the river Danube is the natural (or physical) border between my country Bulgaria and Romania (Europe, Balkan Peninsula).
There are also political borders, built by the humans themselves (look at the straight lines on the map separating Nevada from Utah, Libya from Egypt etc.)
From the famous borders-fortifications I will mention
The Berliner Wall (
http://www.dailysoft.com/berlinwall/)
and the Great China Wall (
http://www.crystalinks.com/chinawall.html).
There is a lot of info about these on the net or in the library, I’m sure you will enjoy doing your homework!
Good luck!