It was a little over a month ago that Bradley Horowitz took over the leadership role of Google's social network, Google+, from Dave Bresbis. In confirming his new position Horowitz mentioned "Google’s Photos and Streams products" and didn't mention Google+ itself. This led to a lot of speculation (for examples see these posts from Pocket Lint , SlashGear, and TechCrunch) that Google was going to break up Google+ by spinning off Photos and Hangouts as separate services.
Collections is a Pinterest-esque way of grouping your posts by topic. Once you create a collection you can make it public or share it with a group of friends (unless your friends use iOS, because Collections are only available on the web and Android for now. Google says they'll be coming to iOS "later").
Collections were enabled for my account pretty late yesterday so I haven't had a lot of time to play with them, but they seem like they have potential. They're almost like a Community where only one person can post (but many can comment). When you create a Collection you decide who can see it (and you can't change this after the fact) and then you can add a header image, tweak the color and things like that. It's fast and easy to set up a Collection.
I threw together a couple of Collections, a private one containing pictures of my dog (mostly as a test) and a public one for posts from this blog. (Anyone want to take a shot at making me a cover image?) I was happy to see that when I added a bunch of posts at one time Google grouped them together rather than spamming my followers.
You can add your existing public posts to a Collection by choosing Add Post to a Collection from the Options menu at the top right corner of a post. If you don't own the post or if it's a private post you can reshare it to a Collection via the dropdown menu next to the Reshare button.
Like most new features in a social network it'll take a few days or weeks to see if Collections are adopted by users and if they serve a function that people want. It'd be interesting if you could appoint curators to one of your Collections so others could post to it, but I suppose at that point it'd more or less be a Community. In fact one of my overall concerns is that it's going to be confusing for new users to understand the difference between a Collection and a Community.