league很正式，意思是联盟，签协议的那种同盟。
association指代组织间关系，很泛指，例如：
1. The New England team are the worst in the league.新英格兰队是联盟中最差的队。
2.They're in a different league from us.他们与我们不属同一个级别。
3.The employers' association was seen as a counter to union power.雇主协会被看作是工会权力的对头。
