Help! Tried sending bitcoins to myself and they disappeared
What you are seeing is a "change address".  When you send an amount that is less than you originally received, it sends the whole amount, and returns the difference to your wallet as "change".  This change may not be spendable until the transaction is confirmed.  The change address is different each time, which makes it harder for a third party to tell for sure how much is sent to which wallet.  If you are not interested in privacy, you can use a different wallet (such as Electrum) which has an option to not use change addresses -- in that case the change would go back to your original address instead of to a new change address.