What is the difference between an inbound and an outbound oracle?

Inbound moves data from the outside world into the blockchain (e.g. a price feed); outbound moves data from the blockchain out to the world (e.g. triggering a payment). Direction is named from the chain's point of view.

Most early oracle work treats oracles as inbound only. Recognising outbound (also called reverse) as a first-class direction is what completes the pattern framework.