Sign message / verify message

[TomCoin]

can someone explain me what sign message and verify message is used for?

What kind of phising attacks am i warned for? what should i never do to prevent that? (bitcoin-qt client)

[jakecnn]

With signing you can prove that you own the private key of an address and thus are the owner of that address.

[nahtnam]

Sites like blockchain.info make it easy for you to sign message.

[LupinS]

Its easy to do in walet too.

[caspar.holzinger]

You use signing to prove to someone else that you are the owner of a bitcoin address.

The most common purpose to do so is when you pay something to someone else: You use the address from which you sent money to sign a message containing your purpose of the payment. This way, the recipient knows who paid him and nobody else can claim that he made the payment.

For example, a signed message could look like this for such a purpose:
message: "i am forum member xyz. as agreed, i sent you btc0.1 for exchange to USD. please send the USD 75 to bank account ..."
signing address: ...
signature: ...

Signing the purpose of a payment with the sender address is important, because everybody can see your transaction to the recipient (transactions are public) and if someone else knows the recipient, this person could tell the recipient a made-up story why he paid this to him and claim the return (goods, other currency, ...) for the transaction.

[monbux]

Sites like blockchain.info make it easy for you to sign message.

Yep, along with Bitcoin-QT.
Basically, it's to show others you actually own that address, and I guess it would be cool to send messages via bitcoin signing messages.  Gearing up for WWIII 

[kalamajo]

I don't understand this.
 

[monbux]

I don't understand this.
 

What don't you understand about this?
I'm not the best technitian here when it comes to bitcoin, but if you need some help feel free to PM me!