Web annotation means being able to add, modify or remove information from a Web resource without modifying the resource itself.  The annotations are like a layer on top of the existing resource. This annotation layer is visible to other users who share the same annotation system.

Web annotation can be used:

• to rate a Web resource, e.g. by its usefulness, user-friendliness, suitability for viewing by minors.
• to improve or adapt its contents by adding/removing material, like a wiki.
• as a collaborative tool, e.g. to discuss the contents of a certain resource.
• as a medium of artistic or social criticism, by allowing Web users to reinterpret, enrich or protest against institutions or ideas that appear on the Web.
• to quantify transient relationships between information fragments.

(see: https://en.wikipedia.org/wiki/Web_annotation)

Pundit Annotator is the easiest tool for web annotation: it allows you to highlight and comment on any text on any web page. It is designed for users who want to exploit the power of web annotation: students, journalists, and users in general.

Once created, your annotations remain on the page so that they are visible to you and other people.