If you're confused as to which Windows Phones can/should get this update then see my exhaustive reference table (click it for a full size version):
The Creators Update will only be 'officially' pushed to phones that are less than two years old. This is the build that rolled out a week ago to the Release Preview ring. And the build that can be achieved by any 'unsupported' W10M-capable phone via the same ring.
Microsoft had originally promised that the Creators Update would start rolling out on April 25th 2017, but I believe this was only true for one phone variant in the USA. So no promises were actually broken, though I'm not at all surprised to see the extra week taken to hit other world regions.
In terms of what's new in the Creators Update (over and above the Anniversary Update, from last summer), see my breakdown here. The big two bullet points being the overarching performance improvements and the big new version of the Edge browser - more and more is now being asked of this, not least because many niche/boutique apps aren't available for Windows 10 and so any interaction with many banks/shops/services has to be within the browser (in fairness, this is already default behaviour for many on the desktop/laptop, but it's going to become more common on mobile).
It's something of a cliché to say that this is the version of Windows 10 (Mobile) that should have launched in 2015, since that sort of thing is always said in hindsight, but in some ways it's true here. Almost all the 'holes' in Windows 10 Mobile are now filled in, the UWP applications are feature packed and mature, there are precious few bugs left that impact day to day use, and - just as it has become hard to actually buy a Windows 10 Mobile phone - the OS is finally ready for the prime time. Ironically.
Which means that the changelog above will be of initial use only to the seven or so million users of the OS across the world - though hopefully future devices and initiatives from Microsoft and others will bring new life to the platform (Windows 10 Mobile already is 'Windows on ARM', but a fuller implementation with Win32 emulation thrown in should debut within a year).
Once updated (via production status or the Release Preview Insiders ring), you may wish to safely 'refresh' your Windows 10 Mobile-running smartphone, especially if you're seeing any glitches caused by jumping between different OS branches. Save this as a last resort though.
If your smartphone is currently on (ye olde) Windows Phone 8.1 and you think that it's capable of running Windows 10 Mobile right up to the Creators Update (1GB RAM minimum), then see also my article on how to upgrade smoothly from Windows Phone 8.1 to Windows 10 Mobile.
Phew! That should cover things by now. If this is all relevant to you and your phone, then you may find the Creators Update waiting for you in 'Settings/Update and Security/Phone update'. Let us know how you get on!