The main information provided by a feature is the set of OSGi bundles that defines the application. Such bundles are URLs pointing to the actual bundle jars. For example, one would write the following definition:
<bundle>http://repo1.maven.org/maven2/org/apache/servicemix/nmr/ \\ org.apache.servicemix.nmr.api/1.0.0-m2/ \\ org.apache.servicemix.nmr.api-1.0.0-m2.jar</bundle>
Doing this will make sure the above bundle is installed while installing the feature. However, Karaf provides several URL handlers, in addition to the usual ones (file, http, etc...). One of these is the Maven URL handler, which allow reusing Maven repositories to point to the bundles.