What is a manifest and when is it appropriate to use one?

Question

I hear the usage of manifests in various ways but never have had the need—to my knowledge—to use one. What are they and how are they used?

EDIT: I'm curious about manifests in general with any language.

Answer 1

A manifest is a blob of metadata that describes the assembly itself (name, version, required external assemblies, etc.).

An assembly must contain an associated manifest (also referred to as assembly metadata). The manifest documents each module within the assembly, establishes the version of the assembly, and also documents any external assemblies referenced by the current assembly

Answer 2

The manifest is a special file that can contain information about the files packaged in a JAR file. By tailoring this "meta" information that the manifest contains, you enable the JAR file to serve a variety of purposes.

http://download.oracle.com/javase/tutorial/deployment/jar/manifestindex.html