Basically blocks of concrete rigged with dynamite at the bottom, once they go off, block tumbles on the road and creates an obstacle for tanks or other vehicles. They are all over Korea from what I have been told although in the last few years, it seems people have asked and received permission to remove them. They also come in different types and can be seen sometimes in conjunction with concrete blocks in a field or stream to prevent tanks from going around the trap. Very common to also spot firing positions near them.
Military - Misc 
http://www.webpages.uidaho.edu/~pzhu/tanktraps/tanktraps.html
Basically blocks of concrete rigged with dynamite at the bottom, once they go off, block tumbles on the road and creates an obstacle for tanks or other vehicles. They are all over Korea from what I have been told although in the last few years, it seems people have asked and received permission to remove them. They also come in different types and can be seen sometimes in conjunction with concrete blocks in a field or stream to prevent tanks from going around the trap. Very common to also spot firing positions near them.
This is a North Korean one:
http://www.flickr.com/photos/54238102@N03/5021134111/