For example ZF is a bunch of code that is written in PHP and runs on Apache (assuming web server...)
SOLR is something that runs on Lucene? that is written in Java and runs on some servlet container.
What is that "something"? What is SOLR to Lucene?
What function does Lucene by itself gives me?
Lucene is a Java library packaged as a JAR that perform the operations of information indexing and retrieval. SOLR is a standalone full text search application that provides features like hit highlighting, and faceted search while using the underlying Lucene library as part of its internal api.
Lucene by itself give you the means through a Java API to index documents independent of file format and retrieve them, but it doesn't provide alot of the supporting functionality that a full featured web based search application requires, SOLR helps fill this gap. Indexing and searching in SOLR is typically done through GET and POST making it easier to use as a piece of infrastructure for web apps regardless of what language they're written in.