Maven consists of a core engine which provides basic project-processing capabilities and build-process management, and a host of plugins which are used to execute the actual build tasks.
"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."
Maven 2.0 is similar to its predecessor in that it has two main functions. First, it organizes project data into a coherent whole, and exposes this data for use within the build process. Second, Maven marshals a set of plugins to do the heavy lifting and execute the actual steps of the build.
Many things in Maven 2 will have at least superficial familiarity to users of Maven 1, and the plugin system is no exception. Maven 2 plugins appear to behave much as their 1.x counterparts do. Like 1.x plugins, they use both project information and custom-defined configurations to perform their work. Also, Maven 2 plugins are organized and executed in a coherent way by the build engine itself - that is to say, the engine is still responsible for organizing and fulfilling a plugin's requirements before executing the plugin itself.
Operationally, Maven 2.0 should feel very much like a more performant big brother of Maven 1.x. While the POM has definitely changed, it has the same basic layout and features (with notable additions). However, this is where the similarity ends. Maven 2.0 is a complete redesign and reimplementation of the Maven build concept. As such, it has a much different and more evolved architecture - at least to our minds. ;-)
However similar the architectures may seem, Maven 2 offers a much richer environment for its plugins than Maven 1 ever did. The new architecture offers a managed lifecycle, multiple implementation languages, reusability outside of the build system, and many more advantages. Arguably the biggest advantage is the ability to write Maven plugins entirely in Java, which allows developers to tap into a rich landscape of development and testing tools to aid in their efforts.
Prior to Maven 2.0, the build system organized relevant plugins into a loosely defined lifecycle, which was determined based on goal prerequisites and decoration via preGoals and postGoals. That experience was critical for the Maven community. It taught us that even though there may be a million different build scenarios out there, most of the activities in those builds fit into just a few broad categories. Moreover, the category to which a goal fits serves as an accurate predictor for where in the build process the goal should execute. Drawing on this experience, Maven 2.0 defines a lifecycle within which plugins are managed according to their relative position within this lifecycle.
Starting with Maven 2.0, plugins implemented in different programming or scripting languages can coexist within the same build process. This removes the requirement that plugin developers learn a particular scripting language in order to interact with Maven. It also reduced the risk associated with the stability or richness of any particular scripting language.
Also starting with Maven 2.0 is an effort to integrate multiproject builds directly into the core architecture. In Maven 1.x, many large projects were fragmented into smaller builds to sidestep issues such as conditional compilation of a subset of classes; separation of client-server code; or cyclical dependencies between distinct application libraries. This in turn created extra complexity with running builds, since multiple builds had to be run in order to build the application as a whole - one or more per project. While the first version (1.x) did indeed address this new multiple projects issue, it did so as an afterthought. The Reactor was created to act as a sort of apply-to-all-these function, and the multiproject plugin was later added to provide Reactor settings for some common build types. However, this solution (it is really only one solution, plus some macros) really never integrated the idea of the multi-project build process into the maven core conceptual framework.
See the previous section for the long version, but the short version can be summed up by the following list of benefits.
In Maven 1.0, a plugin was defined using Jelly, and while it was possibly to write a plugin in Java, you still had to wrap your plugin with some obligatory Jelly script. An XML-based scripting language which is interpreted at run-time isn't going to be the best choice for performance, and the development team thought it wise to adopt an approach which would allow plugin developers to choose from an array of plugin implementation choices. The first choice in Maven 2 should be Java plugins, but you may also use one of the supported scripting languages like Beanshell.
To summarize, the development team saw some critical gaps in the API and architecture of Maven 1.0 plug-ins, and the team decided that addressing these deficiencies was critical to the future progress of Maven from a useful tool to something more robust.