Many materials are copyrighted. They cannot be used freely, for example in a blog post or in a classroom. There is some tolerance regarding the partial use of copyrighted material in such environments, but they are not always clear. They might leave room to interpretation, and you end up forgetting what they are.
To be on the safer side, the best practice is to use material in the Public Domain, or under a “Free Licence” such as GNU Free Licencing or Creative Commons. For digital media such as songs, videos, pictures, online classes, etc., Creative Commons is the most widely used. There are different kinds of Creative Commons licences, which allow you to do different things with the content. Here they are in a nutshell:
But wait… Am I allowed to use this image on my blog? Look at the upper-right corner: you’ll find the Creative Commons Licence. This one tells me I have to attribute it, that I can’t make money with it, and that the resulting work (this blog post) must be under the same type of licence. This image is courtesy of Shiham Donnelly, www.sharingame.org, under Creative Commons licence.