This package is an OO-abstraction to the SQL-Query language, it provides methods such as setWhere, setOrder, setGroup, setJoin, etc. to easily build queries. It also provides an easy to learn interface that interacts nicely with HTML-forms using arrays that contain the column data, that shall be updated/added in a database. This package bases on an SQL-Builder which lets you easily build SQL-Statements and execute them.
You can find some useful usage examples and docs in the MDB_QueryTool documentation. Everything there holds true for DB_QueryTool too, except for the class name, of course.