Many people new to Lucene and Solr will ask the obvious question: Should I use Lucene or Solr?
The answer is simple: if you're asking yourself this question, in 99% of situations, what you want to use is Solr.
A simple way to conceptualize the relationship between Solr and Lucene is that of a car and its engine. You can't drive an engine, but you can drive a car. Similarly, Lucene is a programmatic library which you can't use as-is, whereas Solr is a complete application which you can use out-of-box.
Apache Solr is a web application built around Lucene with all kinds of goodies.
It adds functionality like
Unlike Lucene, Solr is a web application (WAR) which can be deployed in any servlet container, e.g. Jetty, Tomcat, Resin, etc.
Solr can be installed and used by non-programmers. Lucene cannot.
Yes! The Solr community is very vibrant and helpful.
Since Solr uses Lucene under the hood, Solr indexes and Lucene indexes are one and the same thing.
There is technically no such thing as a Solr index, only a Lucene index created by a Solr instance.
If you need to embed search functionality into a desktop application for example, Lucene is the more appropriate choice.
For situations where you have very customized requirements requiring low-level access to the Lucene API classes, Solr may be more a hindrance than a help, since it is an extra layer of indirection.