Ask HN: What's the difference between the "new iPad" & the "iPad with Retina"? 14 years ago
The "iPad 3" did have a retina display; the one that went on sale today is the successor to that version.
Differences that I can remember off the top of my head include a faster processor, better global LTE support, an improved front-facing camera, and the use of Apple's new Lightning connector instead of the older 30-pin connector.
These differences are all internal, so the same cases should work for both models.