Explain 'cocktail attire' please ?

I know about cocktail dresses, but apparently there is a whole spectrum of deportment associated with it that I've never twigged with. I'm not a formal dressing person usually.

Apparently every woman has a LBD (little black dress) but does that 'fit' into 'cocktail wear'? I'm not sure it does as LBD's are for dressing slutty.