REQ: wats da difference between a scale and a key?

hi guys!!! this is a question dats been in my mind since a long time. wats da difference between a scale and a key?? or are both da same?? i need some clarification on this.
i hav heard dat some instruments eg the clarinet play in da key of B flat. does an instrument hav its own key?? n is it supposed to play only in that key only. plz guys i need some clarification regarding wat exactly a key is. another example is the horn which plays in the key of F.
plz clarify.

thanks milesfast................