Zocalo is Amazon's cloud-based file system, like Dropbox or Google Drive.
Amazon S3 is a storage system. You put files up there and can access them either publicly over the web or privately through an API, or both. It's cheap, highly available static storage, and has been around a long time and works well, it's in a lot of developers' toolkits. I use it in all my stuff. This page you're reading right now is coming from S3.
If the two were combined, if you could access Zocalo storage from S3, either reading or writing, you'd have a space that is shared by users, real end-users who are not developers, people who generally can't or won't get over the hurdles to become an S3 user. They would have all the benefits of using S3 without of the complexities.
We already have some of that with Dropbox. But there's a key feature S3 has that Dropbox does not, the ability to put domain names on S3 locations. So we can make it simple to access as well, and the users can control the content. If they want to get off S3 someday they can do it, just by redirecting the domain. And short domain names are much simpler than the long Dropbox URLs which look very fragile. Here's an example.
So while Zocalo is far from the most popular cloud-based file system, it has a feature none of the others can offer, access through the S3 APIs. I asked when the service was announced in July, both privately and publicly, but never found out. I hope the answer is yes, or if not, they're working on this. It would be incredible.
Alternatively, I would totally welcome any of the other vendors in this space to do the same. But I think compatibility with the S3 API is central. There's a huge installed base of know-how. And the API is quite stable.
Or, Dropbox letting us put domain names on folders, that would also be killer.