Copyright rules for publishing is different from teaching. In publishing, the main issue is distribution. A writer's goal is to distribute his/her work as much as possible to gain a wide range of readers for the work. Publishing can be either commercial or non-commercial.
There is no difference between publishing in print or online from the copyright point of view. All copyright rules apply to web pages, blogs and articles online the same way they are applied to print articles in journals and books. The only major difference is in online works in which the author has the option to link to a copyrighted work. Remember that linking is like giving an address. There is no permission needed from copyright holders to give the link to their materials. But if you want to download copyrighted materials in your work instead of giving the link to them, you need to get permission from copyright holders of those materials. In publishing in print you always need to get permission if you're using copyrighted works such as images, pictures, and extensive length of texts.
Jane Friedman's article: "A writer's guide to permissions and fair use" is an excellent guide for copyright issues related to publishing.