PartSkeptic
Illuminator
I'm sure that there is plenty of info out there if you want the nitty gritty details (if memory serves me, it had something to do with faulty encryption).
Suffice to say that if you transfer bitcoins from your personal wallet to the exchange's wallet then the security of your bitcoins is, at most, that of the bitcoin exchange's.
I may be wrong but I think that much of the money paid to the exchanges does not properly get into the bitcoin block and give the person confirmed direct ownership. Too cumbersome - a major weakness.