Copyright applies fully and automatically to any work a photograph, a song, a web page, an article, pretty much any form of expression the moment it is created. This means that if you want to copy and re-use a creative work you find online, you usually have to ask the author’s permission.
Some authors, myself included, want people to use their content freely and without the need to ask – under some conditions. The Creative Commons non-profit organization allows authors to mark their work with “some rights reserved” and then partner with CC to index this content at the Creative Commons search engine.
By doing a search at the Creative Commons search engine you can turn up vast amounts of content on very specific topics, most of which is free to use with little restriction.