Andrew Cunningham, for Ars Technica:
[Today, Apple] is releasing the first of many Safari Technology Preview builds, a new development snapshot of the browser designed to show off more features while offering more reliability and stability than the nightly WebKit browser builds.
I love this new Apple.
Moreover, as John Gruber notes, since these builds are signed by Apple, users will have iCloud Tab Syncing—a feature not available in the WebKit Nightlies.