The extra information associated with a particular point in a document or other piece of information. Most commonly this is used for example in draught documents, where another reader has written notes about the quality of a document at a certain point, "in the margin". Annotations about bibliographical sources, labelled annotated bibliographies, give descriptions about how each source is useful to an author in constructing a paper or argument. Creating these blurbs, usually a few sentences long, establishes a summary for and expresses the relevance of each source prior to writing.

In computing, the programmer often adds annotations to source code in the form of comments. These do not affect the working of the programme but give explanations (for their programmers, or potential readers of the code principally, but also as a reminder for the author), hints or plans for improvement, etc.

Further annotations can also be added by a compiler or programmer in the form of metadata, which is then made available in later stages of building or executing a program. For example a compiler may use metadata to make decisions about what warnings to issue, or a linker can use metadata to connect multiple object files into a single executable. Differences in computer languages have given rise to a variety of words for programmer-added metadata, including annotation (Java), attribute (C#), pragma (C), and metadata (HTML).