expression rule vs decision

Hi,

     I know decision designer is new feature.

     Would like to check what are best use cases for writing expression rules  and  best use cases write decision rules? 

     Where are these decision rules are going to stored. Do we have access these tables         

      Can we group decision rules under one rule set and make use of them in another rule?  Can we use this decision designer as a typical rule engine(IBM ODM,DROOLS,PEGA rule engine) functionality?

Thanks.