Nokia has yet to talk in detail about what's in Lumia Cyan, but during his appearance in the Build keynote Stephen Elop made references to improvements in imaging software and experience.
There will be updates for Nokia Creative Studio (new filters, non-destructive editing, and more), plus a new and improved version of the Nokia Storyteller app, with support for "living" images (what appears to be short animations).
Nokia's updated HERE app for Windows Phone 8.1, as detailed here, can also be considered to be part of the Lumia Cyan update. The key new features here are updated aerial imagery, extended support for 3D buildings, and HERE Maps specific gestures for map browsing and rotation.
The update will also likely include enablers for Nokia's new SensorCore SDK / technology (always-on motion sensor, available on select devices). However, the main emphasis here will be on getting third party developers to build new (or update existing) apps around the SensorCore platform, though we assume Nokia will provide some leadership here (e.g. integration with Bing Health & Fitness app).
Nokia will shortly be holding its #moreLumia event to share more details on the Lumia 630, 635 and 930. We expect further details of Lumia Cyan will also be revealed.
Update: No major revelations, but we'll do some more digging tomorrow.
The Lumia Cyan update will be delivered to existing devices in the "coming months" and as with other updates will be a staged roll out, with some devices and variants getting the updates before others. Lumia Cyan will be first seen on the Lumia 630, which will go on sale in early May. The Lumia 930 will also ship with Lumia Cyan pre-installed.