An annotation is a note that is made while reading any form of text. This may be as simple as underlining or highlighting passages.

Annotated bibliographies

Annotated bibliography

An annotated bibliography is a bibliography that gives a summary of the source.The purpose of annotations is to provide the reader with a summary and an evaluation of the source. In order to writes a successful annotation, each summary must be concise...

 give descriptions about how each source is useful to an author in constructing a paper or argument. Creating these comments, usually a few sentences long, establishes a summary for and expresses the relevance of each source prior to writing.

The term also has a special meaning in a number of other fields.
In programming, annotations are used mainly for the purpose of expanding code documentation and comments.