What is it?
Hundreds of thousands of images are available, and none of them require attribution (although you are still certainly welcome to do so).
Images can be downloaded as files and then used as you would normally.
Is there a catch?
Not really. However, but you will notice that typically the first row of images are "Sponsored," which means that those do cost money.
You will notice that you can signup and login, but you don't need to do that to use the site.