I'm not a native English speaker, so this is definitely not intuitive for me. I googled for it to see if I could find anything better than what has been said in previous answers. http://answers.yahoo.com/question/index?qid=20090726110105AAesLvw Here it says more or less that: Yelling is often aggressive, caused by anger. Shouting would often be purposively making a lot of sound to ensure you'll be heard (in a noisy environment, or over a large distance), but it can be reflexive too. This answer fits with how I remember "yell" and "shout" being used in stuff I've read and heard.

Yell and Shout are synonymous but there are regional differences when used as expressions. For example: "With a Rebel Yell she cried 'More, more, more!'" and "Shout! Shout! Let it all out!"" "Stop yelling at me!" "They were having a shouting match." They are also used commonly as euphemisms. People may tell us "When you are ready, give us a shout [yell]"; not literally wanting a loud conversation, just to be sure that you get our attention.

I believe they are synonymous my friend but different in degree. Yell is a bit worse than shout. :)