"Maven" is really just a core framework for a collection of Maven Plugins. In other words, plugins are where much of the real action is performed, plugins are used to: create jar files, create war files, compile code, unit test code, create project documentation, and on and on. Almost any action that you can think of performing on a project is implemented as a Maven plugin.
Plugins are the central feature of Maven that allow for the reuse of common build logic across multiple projects. They do this by executing an "action" (i.e. creating a WAR file or compiling unit tests) in the context of a project's description - the Project Object Model (POM). Plugin behavior can be customized through a set of unique parameters which are exposed by a description of each plugin goal (or Mojo).
One of the simplest plugins in Maven 2.0 is the Clean Plugin. The Maven Clean plugin (maven-clean-plugin) is responsible for removing the target directory of a Maven 2 project. When you run "mvn clean", Maven 2 executes the "clean" goal as defined in the Clean plug-in, and the target directory is removed. The Clean plugin defines a parameter which can be used to customize plugin behavior, this parameter is called outputDirectory and it defaults to ${project.build.directory}.
A Mojo is really just a goal in Maven 2, and plug-ins consist of any number of goals (Mojos). Mojos can be defined as annotated Java classes or Beanshell script. A Mojo specifies metadata about a goal: a goal name, which phase of the lifecycle it fits into, and the parameters it is expecting.
MOJO is a play on POJO (Plain-old-Java-object), substituting "Maven" for "Plain". Mojo is also an iteresting word (see definition). From Wikipedia, a "mojo" is defined as: "...a small bag worn by a person under the clothes (also known as a mojo hand). Such bags were thought to have supernatural powers, such as protecting from evil, bringing good luck, etc."
What is the Build Lifecycle?The build lifecycle is a series of common stages through which all project builds naturally progress. Plugin goals are bound to specific stages in the lifecycle.
Maven covers the main build phases of a project separated in 3 lifecycles: default, clean and site.
Clean Lifecycle