I guess it was technically available to everyone already, but you needed to have a Google Apps account to use it. Now, as of today, anyone can simply create a "site" on the relatively new Google service.
A few months ago we launched Google Sites exclusively as part of Google Apps for companies and organizations that wanted to use the service on their own domains. Now we've made it easy for anyone to set up a website to share all types of information -- team projects, company intranet's, community groups, classrooms, clubs, family updates, you name it -- in one place, for a few people, a group or the world. You can securely host your own website at http://sites.google.com/[your-website] and add as many pages as you like for free. -- Official Google Blog
Now that this service publicly available, you may wonder what the difference between Google Sites and Google Pages is. Well, you can technically use both to create a public website, but Google Sites is geared more towards collaborative, private web portals than a traditional website.
[image from Google]
The reason Google Pages is better suited for external websites (like for instance a personal webpage) is because it can be hooked up to your own domain, and overall, you have better control over its appearance.
Google Sites, on the other hand, gives you much more power as far as collaboration goes, but you have to use a special website address, and many things like the search bar at the top of every page are not optional. A "Google Site" can be thought of as more of a wiki where people you give access have the ability to easily update the content.