Here's what their website has to say about it:
"All photos published on Unsplash are licensed under Creative Commons Zero which means you can can copy, modify, distribute and use the photos, even for commercial purposes, all without asking permission."
They post 10 new free photos every 10 days. There is no search capability and no categories. So you just have to start scrolling through to find images that catch your eye. But then, that's half the fun! And there are some really good quality photos there.
You can also subscribe to their email notifications so you know when new images become available.