CPDLC is a data link application that allows for the direct exchange of text-based messages between a controller and a pilot.
CPDLC greatly improves communication capabilities in oceanic areas, especially in situations where controllers and pilots have previously had to rely on a Third Party HF communications relay.
The following link provides a high-level overview of CPDLC. Note - when the presentation loads, click on the word "More" to the right of the start button to select full-screen mode.