The old Lucene query parser used to have only one class that performed all the parsing operations. In the new query parser structure, the parsing was divided in 3 steps: parsing (syntax), processing (semantic) and building.
The classes contained in the package org.apache.lucene.queryParser.standard are used to reproduce the same behavior as the old query parser.
Check org.apache.lucene.queryParser.standard.StandardQueryParser to quick start using the Lucene query parser.
There are 2 wrapper classes that extends QueryParser and MultiFieldQueryParser. The classes implement internally the new query parser structure. These 2 classes are deprecated and should only be used when there is a need to use the old query parser interface.