I have never heard the term Sybil attack. I presume in the case of bitcoin a Sybil attack would be having your private key stolen?

The term refers to subverting a reputation system by creating loads of spurious participants. See https://en.wikipedia.org/wiki/Sybil_attack for details.

With regards to Bitcoin: The value in a distributed block chain is to ensure participants don't double-spend their coins. An attacker in a Sybil attack would spend their coins, broadcast the transaction and (after the recipient believes he's received the transfer) use fake identities to "vote against" including said transaction in the public ledger. That way, he could then go on to spend the coins again. To prevent this, Bitcoin uses a proof of work function: each participant in the network gets a share of the voting power corresponding to the effort he or she spends on calculating sha256 hashes, not to the number of identities one represents.

> To prevent this, Bitcoin uses a proof of work function: each participant in the network gets a share of the voting power corresponding to the effort he or she spends on calculating sha256 hashes, not to the number of identities one represents.

Which leads to the problem that whoever can spend the most money on computing power gets the most votes.

I never understand why some people can't use Google and choose to publicly announce their ignorance.

I never understand why some people always insist on writing the pointless why did you not just Google it response. I am also not convinced that he did not Google it and was asking about clarification about the parents intent. Furthermore not everyone thinks publicly admitting you don't know everything is shameful.