lucene/ is a search engine library
solr/ is a search engine server that uses lucene
modules/ is shared code

To compile the sources run 'ant compile'
To run all the tests run 'ant test'
To setup your ide run 'ant idea' or 'ant eclipse'

For more information on how to contribute see:
http://wiki.apache.org/lucene-java/HowToContribute
http://wiki.apache.org/solr/HowToContribute