What is going on?

[Kazu]

So I'm in Electrum. I want to send a small amount to 1t4v2HfVwWNEDUV116zV8W1dmLmnJ3Cww for a test. I keep on getting an error along the lines of 'you need to pay a bigger fee' (which made no sense, on my previous client this wasn't happening from what I recall). In any case, I end up paying a fee and sending the payment. My Electrum now shows "79584e3b513a088959a7e5cf44ee678f70e3da05c24c646572603a1e8a632e5f" with -0.0012 unconfirmed. (0.0002 was the fee). Now, I go onto blockchain.info, and enter that in. I see a transaction to "1J2qpiJiF2HWUaJxndwJBdAGz2QQixq4Zr" for the amount of "0.6400656", which is the remainder of my account balance in Electrum? What? Where did that address come from!

Can somebody explain to me what is going on here? Was I somehow hacked, did I mess up epicly, or is this somehow to be expected and the address 1J2qpiJiF2HWUaJxndwJBdAGz2QQixq4Zr will get the BTC back to me?

[BlindMixerDR]

Most bitcoin clients always send the change from a transaction to a new address. That is to say, the client creates a new private key (with address) and sends any extra bitcoins from the transaction to that address. The 0.6388656 BTC are still in your wallet and you can still spend them.

[odolvlobo]

Fees:

Transaction fees are completely optional; however, paying or not paying a fee can affect how quickly the transaction is processed. Most developers of the various clients feel that fees should be or need to be paid in certain cases, so they put that into their clients. Different clients (and versions) can handle fees differently.

Mystery addresses:

In very simple terms, when you send any bitcoins from a particular address, all the bitcoins at that address must be sent somewhere. This is how bitcoin works. The client automatically creates a new address (your mystery address) to send the extra bitcoins (called "change"). Your wallet automatically keeps track of these addresses for you. Clients generally keep them hidden since most people don't really need to know about them.

For example, if address X has 100 BTC and you want to send 1 BTC to address Y, your client will create a new address Z and send 1 BTC to Y and 99 BTC to Z. You now own addresses X and Z, so you have 99 BTC.