An extended private key is a format.
A derived private key is any private key that is deterministically generated from another private key.
Any private key can use the extended private key format to give wallets information about how to derive that key given the master private key that it was derived from.
So, which key do I use to import into Electrum or something?
To import a deterministic wallet into Electrum, you would use a private key in the extended private key format encoded with the base58 check encoding.