The purpose of the Core Shield Generators is to provide a more multi-stage game, where it's not all-or-nothing at the AI home planets alone. It also makes you take a bit more territory than some players might have previously done, but for many players it actually won't impact their playstyles at all. For details, be sure to check out the release notes, which also include a lot of the rationale behind that.
On the flip side, the AI waves have had yet more fixes, and so the schizophrenic waves aren't so overblown as they recently have been, even in the prior release.And beyond the above, a pretty huge array of bugs and balance tweaks have been put in. It's several page's worth, so pretty difficult to summarize.
Oh, and there is a new "Mobile Military (Firepower)" filter option in the galaxy map. That's a pretty cool addition that makes it much easier to size up your forces versus an AI opposing force. Unit counts only go so far, especially with all the supersized units around these days.
This is a standard update that you can download through the
in-game updater itself, if you already have 4.000 or later. When you
launch the game, you'll see the notice of the update having been found
if you're connected to the Internet at the time. If you don't have 4.000 or later, you can download that here.