Chances are, you've seen a Creative Commons license. These licenses enable copyright holders to provide preemptive permission for certain uses. There are actually six difference CC licenses composed of various rights and restrictions. All of the CC licenses require attribution, and best practices for attribution are provided. You can also use the Open Attribute tool to produce a properly formatted attribution that can be copied and pasted.
"Creative Commons Licenses Explained" by MaconEastLibraryProject is licensed under CC BY-SA 3.0