how apache works - Wikitechy

Indexing

Apache Solr is capable to get quick search responses because, as a replacement for searching the text directly, it searches an index instead.
It stores index in a directory called index in the data directory.

The Apache Solr creates inverted index of the documents that are posted in to its collection or core.
Core or collection is just part entity which can be used to post documents and running queries on that set of documents.
Inverted index is a main term in the domain of Information Retrieval and Natural Language Processing.
Now take a document, note down all the unique words appearing in that document as well as note the frequency of the words. Hence the inverted index is ready.
Solr creates similar inverted index of the documents posted to its core using a defined schema.
Schema is a blue print which helps Solr in creating inverted index of the documents by giving a set of predefined fields in the schema.xml file.
Once Solr completes indexing the documents posted to its core or collection it can be used to run queries.

Now a user can query Solr for related documents using its query parser or admin panel.
When a query is given to Solr, it breaks the query into different chunks or entities and matches it with the inverted index of the documents created previous.
The documents obtained accordingly of a Solr search is based on the same class and other constraints defined in its schema.xml and solr.config file.