Um, the standard is 0db.  You achieve that by turning up the master gain until you're as close to 0db without any clipping.  

Then there's relative loudness which is how loud it sounds when maxing out at 0 db.  For that you NEED to use some sort of compression/limiting.  The industry standard seems to be as loud as possible until it really sounds like poop.  That way your label's material stands out the most.  The first releases of Metallica's "Death Magnetic" were made so "hot"  (as in relatively loud), that they had to be remastered after listeners complained that their speakers were defecating.  Even though it sounded louder than anything else, it still peaked at 0db.  Going over 0db means the speakers will actually distort the sound.  Going less means you could have been louder.

The industry standard seems to be as loud as possible until it really sounds like poop.