Grant Ingersoll at IBM developerWorks – What's new with Apache Solr:

"With the 1.3 release, Solr adds in distributed search capabilities. The application splits up the documents across several machines, commonly referred to as shards by Solr (and others). Each shard contains its own self-contained index, and Solr can coordinate the querying of the indexes across the shards. Unfortunately, at this time, applications must still handle the process of sending the documents to individual shards for indexing, but this will likely be added in a future Solr release.