Determining D and L Stereoisomers: Explained

[pzona]

I was wondering about this earlier today: how do they (IUPAC?) decide which stereoisomer is D and which is L? I understand the difference between the two, but I don't know how to determine which is which. I imagine it has something to do with chiral priority, but could someone explain, or give me a resource on this?

 

[Borek]

http://en.wikipedia.org/wiki/Chirality_(chemistry)#Naming_conventions