word usage - What is the difference between "yell" and "scream"?

I am having some problems to differentiate between "yell" and "scream".

From what I understand (and read), when somebody screams he/she is shouting in a loud way. Then, yelling is the same, only that it has the connotation of shouting out of control.

Am I right? Is there any piece of context I am missing?