what is the difference between idiom and slang

I have long been a bit confused with the difference between these two words, "idiom" and "slang" , are they interchangable, etc.. even though I do have a hunch of their difference. Just googled to make it clear in my mind once and for all...

Here are couple of the links to explain their difference... 

http://www.differencebetween.com/difference-between-idiom-and-vs-slang/

http://ca.answers.yahoo.com/question/index?qid=20060810161833AAUWUzL