WD HDDs come in three "quality" flavors: Black, Blue and Green.
Black - Maximized performance
Green - Optimized for less power-consumption
Blue - Balance between power consumption and performance.
Basically, if you want out and out performance, Black is the way to go. If you want just to store data (as in movies, music, etc), Green will save some money. Blue is a more cost-effective performance drive, although not as good as Black.
Blacks and Blues are 7200 rpm. Green is probably variable - but runs at 5400 rpm normally.