In short, it used to be a fulltext search through the database, and now is is Solr built on Lucene.
However, I think PG's essay about why he doesn't have a better search on HN applies equally to reddit -- because there are much better things to spend time on.
Is there some reason that using Google isn't good enough? Most of my trouble searching reddit is because it's hard to translate "that picture of a guy with a bacon helmet" into something Google understands.
http://www.reddit.com/help/search
In short, it used to be a fulltext search through the database, and now is is Solr built on Lucene.
However, I think PG's essay about why he doesn't have a better search on HN applies equally to reddit -- because there are much better things to spend time on.