REST) is the new read/write API that CCP is developing. A replacement for the current read-only API, which only supports EVE Online. The new API is being written to support the new Two Games // One Universe model.
When Crest will be released, only CCP knows (and maybe the CSM, kinda.) But soon, hopefully. Maybe it will be part of the Winter Expansion.
The most exciting part of the new API and its adoption by third-party apps (like my favourites, Aura for Android and EveMon for Windows) will be the ability to set-up skill queues tens and hundreds of skills long, and then let the third-party app manage it automatically. The app can then insert new skills into our queue as space in the 24-hour window opens.
Unless CCP decides not to enable skill queue writes, there's nothing they can do, programmatically, to prevent third-party apps from implementing this sort of automated skill queue management. Short of sending third-party developers nastygrams telling them to stop on threat of denying them access to the API system, this is going to be the most immediate change that will impact EVE players in a noticeable and positive manner.
Do I think CCP will deny skill queue writes? Or that they will try to force third-party developers from implementing automated skill queue management features? Not likely. Perhaps three or fours years ago, they might have, seeing that automation of this sort really steps outside of what the client itself allows. The new design and development paradigm within CCP, since Crucible, seems to be to give players the convenience they desire, without sacrificing complexity. The task of creating viable and meaningful skill queues for ourselves remains unchanged, we'll simply be allowed to set up good skill queues that can continue unabated over holidays and vacations. No more of this "Oh no! I'm going on holidays for four weeks, I'd better interrupt what I'm currently doing to throw on a 5 week skill that I don't really need yet."
The upcoming CSM Summit novel, Tectonic Rendezvous, due very soon, should have quite a few pages devoted to Crest talk.