Validation

[magie720]

How does bitcoin validate that a peer has the coins it claims to have? IE, what keeps me from modding the client on my computer to claim I have more than I do?

[indio007]

Non-technical explanation...

The amount of bitcoins has is stored by the entire network. If  what the client transmits don't jibe , the transaction will be rejected

[Stephen Gornick]

How does bitcoin validate that a peer has the coins it claims to have? IE, what keeps me from modding the client on my computer to claim I have more than I do?

 - http://en.bitcoin.it/wiki/How_bitcoin_works#Double_spending

[nafai]

Bitcoin accounts are public/private key pairs. The address you send to is the public key. The private key that matches that public key is necessary in order to spend the bitcoins. You can create as many public/private keys as you want but they all have zero bitcoins unless someone sends btc to the public keys of those accounts. You can't mod your client to give yourself more bitcoins, because the bitcoins are associated with the public/private key pairs, not with the client or wallet.