Comment by elric

In all three cases, Lucene is used as a "low level" (Java) API which provides search capabilities. OS, ES and Solr turn Lucene into a server, with features like horizontal scaling (ES Cluster, Solr Cloud). The major differences are in how well that all works, how easy it is to administer, how much caching and optimization is done on top of Lucene, etc.

I haven't extensively used ES, but I've used Solr a lot (and contributed to), and I can say that it's a mess. The community is not one of the better ones I've seen. Bugs and stability issues are often ignored. Patches sit around gathering dust. There are some gems and very clever people in the community, of course, but it seems like there are too few of them to cope with the large beast that Solr has become. If I were starting a new project in the search space, Solr would not make my shortlist.