WD Blue
These are your baseline drives; typical power consumption, solid performance, good price. If none of the specific features of the other types stand out then these are the ones you want, they're ideal as capacity drives for a desktop for example.
WD Green
These are all about saving energy; they're not actually all that slow in practice for things like streaming, but for more random read/write they lag behind a bit, again not by all that much. The main benefit is that they save power and wear by spinning down when they can, this means they're basically best for things like backup drives which are only in use periodically (e.g - once an hour), if they're made to spin up too often then you obliterate any potential savings you could make, at which point you've got a slightly slower Blue.
WD Black
These are pure performance drives, all about speed. Their top speed for streaming isn't that much further ahead than a blue drive, but the main difference comes from it being generally more responsive. Basically if all you want is speed, but you can't afford an SSD with the capacity you need, then WD Blacks are for you. A good value gaming system can do well with an affordable SSD for OS and a few other bits and pieces you can fit, with a WD Black as your main drive for your games, for example by moving your Steam folder onto it, giving you good all round performance and capacity.
They also now have generous warranties (5 years), they're basically WD Red+, if you can take advantage of the extra performance that is.
WD Red
I think of these as a hybrid between Greens and Blacks; their power consumption is really good, but unlike the Green which is designed to save power between uses, the Reds are designed to just spin constantly for continual use/availability. They're quiet, responsive, and have good speeds, but most importantly they have an extended (3 year) warranty.
You could use them as system drives and they'd perform just fine, but you probably wouldn't be getting the most of their cost; they're ideal for often used NAS devices, I also like them for RAID setups, particular RAID-5 and RAID-6 since a bunch of them doesn't use tons of power, but they're responsive enough to handle the distributed blocks of data, parity writes etc.
So to summarise:
WD Black = Speed/high end use, extended warranty.
WD Blue = General use.
WD Green = Energy saving for backups or other less frequent use.
WD Red = Some energy saving, continual use, extended warranty, ideal for RAID.
Not to say that you can't use them for different things, ultimately they're still all HDDs with good streaming speed etc. and they're close enough that for general use I doubt you'd notice the difference. But basically if you don't need the extra features of the Red and Black then you're wasting money, and you're probably wasting a Green if you don't let it spin down. The prices can vary rapidly though, so sometimes it's worth waiting a while to see if you can a Black or Red at a reduced price (I got some a little while ago for a NAS at only about £5 extra per drive over Greens!).
