With the most recent update to Google+ photos, Google also introduced a new image recognition search technology that it has already used in other products.In practice, this means that you can now search for photos on Google+ based on topics. For example, search for "cars" and you get photos of cars, search for "flowers" and you get photos of flowers.
There's nothing surprising about that, as that's how image search is supposed to work. The surprising thing is what happens under the hood.
Because it uses image recognition, Google can retrieve images of cars or flowers even if there is no reference to either subject in the album name, in the file name, the comments, the tags, and so on.
Where it gets really impressive is when you start searching through your own photos. With image recognition you'll find photos just by searching for what's inside them, no matter what album they're in, or what they're called or labeled.
It's an amazingly powerful feature, and Google's technology has gotten so good that you'll rarely see it mess up. It still has some problems, however. For example, sunlit mountains will be mistaken for snow, stretches of snow for roads, and so on, but it's rare enough that it's not a problem.