Does someone understand this?
"Because of all this work, when a Bitcoin client signs on to the network it can trust the block chain that was most difficult to produce (since this is evidently the one that was being worked on by the most miners). If there was a "fake" blockchain competing with the real ones (say, where someone pretends that they didn't actually give Sofia those 8.4 bitcoins and they still have them), the fraudster would have to do as much work as the whole rest of the network to make their block chain look as trustworthy. So essentially, the intense work that goes into finding blocks through hashing secures the network against fraud. There is also, of course, some nifty code that figures out how to choose between conflicting transactions; and what to do if two people find valid blocks at the same time."