[ropable]

For those of us late to the Bitcoin idea, how does one "steal" Bitcoins? Is it the equivalent of copying someones private key and then deleting all their copies of the key?

[mckoss]

No, you use a stolen private key to transfer the Bitcoins to a new public key whose private key is only known to the thief.

[wmf]

Generally the thief copies the wallet (private keys) and transfers the BTC to his own account. It's not necessary to delete anything.

[marshray]

Or just copying the key and "spending" it before anyone else can.