Memo Key?

[xeroc]

So if I understand correctly:  With any transaction, I can attach a memo, basically a text string?  The receiver &  sender can read the text string, but nobody else?  While the memo is attached to the transaction, the private key that unlocks the memo can't send any coins from the account?

Am I understanding it correctly?

Correct ... memos can only be decoded by sender and receiver.

However, sometimes, active key and memo key are the same! .. in that case a compromised memo key would compromise your funds!

[Sage]

So if I understand correctly:  With any transaction, I can attach a memo, basically a text string?  The receiver &  sender can read the text string, but nobody else?  While the memo is attached to the transaction, the private key that unlocks the memo can't send any coins from the account?

Am I understanding it correctly?

[svk]

It is used to encrypt/decrypt the optional memo that can be attached to a transfer, making it so that only yourself and the receiver can read that memo.

[Sage]

Newbie question:

What is a memo key?  And what are some use cases for it?