The
ApacheSolr module allows the integration of the core search module with the power of the
Apache Solr search platform. Aside from better performance the most prominent feature is the addition of faceted search. In essence it's a user-friendly manner to search for content by filtering on author, content type, taxonomy,... You can try a demonstration on the search pages of
Robert Douglass, author of the ApacheSolr module.
Now, not everyone has the resources to set up a server with Apache Solr, be it an actual dedicated server to run the software on or the funds to rent one. Luckily, there's also the
Faceted Search module that provides similar functionality without this extra requirement. You can check out the demonstration by
David Lesieur and see how it compares to Robert's.
Faceted Search also has a couple of extra features that make it a worthy competitor.