in the example above, "abc123" and "def456" are the bitcoin addresses of bob and alice. but bob and alice each have a second key which only they individually know. this is the private key, and it is the "other half" of a bitcoin address. the private key is never shared, and allows the owner of the bitcoins to control them. however, if the private key is not kept secret, then anyone who sees it can also control and take the bitcoins there. this happened on live tv when bloomberg's matt miller accidentally showed a private key to viewers. the money was taken immediately. the person who took it, told others about it later, saying "i'll send it back once matt gives me a new address, since someone else can sweep [empty] out the old one."