How does Methadone work and what exactly is it ?

I know it's a treatment for heroine but what does it do ? I have noticed users on here asking about it and I have heard about it but I still don't know what it does.

Does it get you high so that you're satified and don't want heroine ?
Is it illegal unless prescribed by a doctor ?
What exactly is it ?

: http://alcoholdrugstreatment.info/how-does-methadone-work-and-what-exactly-is-it