plexus-default-container is Plexus' inversion-of-control (IoC) container. It is composed of:
Default implementation reads configuration in XML files:
But it is not limited to these files: Plexus container is by nature very extensible, it can be configured programmatically too or extended to read configuration from any source. It is used for example in Maven 2 to read plugins configuration from META-INF/maven/plugin.xml and instanciate Mojos downloaded from Maven repositories.