In a Maven POM, the packaging element indicates the package for the generated artifact (e.g. JAR, WAR, EAR, etc.). The default value for the packaging element is jar, so a lot of times this is omitted.

The current core packaging values are: pom, jar, maven-plugin, ejb, war, ear, rar, par. Default goals are bound to Maven build lifecycles by declaring the type of packaging.

Custom packaging types may be defined and made available to Maven. To use a custom packaging type, you need a plugin which defines the lifecycle for the custom packaging type.

Reference:
http://www.sonatype.com/books/maven-book/reference/lifecycle-sect-other-packaging.html