In broad terms, annotation is the process of adding or modifying information in a piece of media in a way that does not alter the media itself. On paper, annotations may take the form of highlighting or adding comments in the margins... but in the online environment, it is possible to annotate web pages, PDF files, images, maps, audio, video, and more.
Unstructured annotation is free-form: readers highlight or comment text using any procedure that they find interesting.
Directed reading is more formal than unstructured annotation, and usually involves an instructor specifying the types of annotations that should be made (for example, noting key words, central ideas, or figures of speech; adding tags).
Collaborative annotation may be unstructured or directed, but takes place in an environment where existing annotations can be shared and used as the basis for replies, additional commentary, and discussion.
free | web-based | easy to learn
free | Windows, Mac, Linux | basics are easy to learn