What is RingCT and how does the addition of Ring Signatures enhance the privacy properties of RingCT compared to Confidential Transactions as designed by Greg Maxwell?
RingCT just like Confidential Transactions hides the amount of each transaction. Unlike Confidential Transactions, RingCT will also make the payments unlinkable.
Confidential Transactions include a cryptographic proof that the sum of the input amounts is the same as the sum of the output amounts, without revealing the actual numbers. You are still able trace which address sent and received BTC.
RingCT protects the privacy of more than just the transaction amounts. Monero transactions are already untraceable thanks to ring signatures and unlinkable thanks to stealth addresses. There are also plans to use I2P to protect IP address with the Kovri C++ router project.