It has been confirmed that the shipping units of the new Apple Watch will contain 8GB of storage. However, that storage is subject to certain limitations.
As a user, you are only allowed to use a subsection of that total space for certain types of data. As confirmed with Apple, users are only allowed to store up to 2 GB of music on the Watch and up to 75 MB of photos for the Photos app. Note that all three collections — Sport, Watch and Edition — feature the same 8 GB memory capacity.
Music and photos stored on the Apple Watch can be played or viewed without needing a connection to an iPhone. This allows those exercising, such as runners, to leave their iPhone at home, and still be able to listen to music. (2GB will allow storing around 200 high-quality music tracks, so it’s enough to get you through your exercise session, but not an entire day’s worth of listening.)
Songs are synced via the Apple Watch companion app, available in iOS 8.2. When the Apple Watch is connected to the iPhone via Bluetooth, users can listen to any song in their music library.
The 75MB photo storage limit is designed to allow you to only store a few favorite snapshots on the watch, and is managed via iCloud Photo Library. Photos will be optimized for the screen of the Apple Watch, and will take up much less room on the wearable device. 9to5Mac estimates around 100 photos can be stored in the allotted 75MB of photo storage.
What is the 6GB of remaining storage used for? Likely a few gigs are used by the operating system, and internal apps, leaving the rest for data like settings and app caches. Once a native SDK arrives some of the space will also likely be used to store app binaries and resources.