Google has released more details about the upcoming Google Photos on their official blog:

Google Photos gives you a single, private place to keep a lifetime of memories, and access them from any device. They’re automatically backed up and synced, so you can have peace of mind that your photos are safe, available across all your devices.

And when we say a lifetime of memories, we really mean it. With Google Photos, you can now backup and store unlimited, high-quality photos and videos, for free. We maintain the original resolution up to 16MP for photos, and 1080p high-definition for videos, and store compressed versions of the photos and videos in beautiful, print-quality resolution. For all the storage details, visit our help center.

And also:

If you want to give Google Photos a whirl, it will be available later today across Android, iOS and the web. With this launch we’ve made a lot of progress towards eliminating many of the frustrations involved in storing, editing and sharing your memories. But we have a lot more in store—so as you keep snapping photos and capturing videos, we’ll keep working on making them even easier to store, share and bring to life.