slang or idiom?

I'm confused...I know what slang and idioms are but sometimes there are phrases that i think are idioms but are actually slang..for instance; "an arm and a leg" it says that it's slang, I've thought that as an idiom.help!

Usually slang denotes idioms that are shared by one particular group.

For instance, burn-out can mean one thing in drag-racing slang, and something else in teachers' slang.

If you're a musician who got a new axe, that's a new guitar.
If you're a businessman who just got axed, then you've been fired.

We give it as an idiom: http://www.usingenglish.com/referenc...and+a+leg.html

Slang is also used for more colloquial language, and would extend to less acceptable language. ;-)

thanks!!: )