The difference between domain.com/folder and domain.com/folder/

I've been told that these both behave different in terms of loading resources on the page

http://ift.tt/UOwI8H
http://ift.tt/1glMDmM

However I've also noticed that no matter what I do, the browser redirect http://ift.tt/UOwI8H to http://ift.tt/1glMDmM

So I wanted to ask -- what's the major difference between the two? What should I do for my browser to not redirect (or add the end slash)