From Oracle FAQ
Oracle Discoverer is an ad hoc query tool that ships with the Oracle Developer Suite. It provides end users easy access to data and allow them to do data analysis and set-up ad-hoc queries.
Oracle Discoverer (like Oracle Reports and Express Objects) is part of Oracle's Business Intelligence set of tools.
Oracle Discoverer consists of the following components:
- User Edition - the graphical interface utilized by users on a day-to-day basis to generate reports and analyze data.
- Discoverer End User Layer (EUL) - metadata layer that shields users from the complexity of the relational database from which Discoverer retrieves data. Conceptually, the EUL resides between the database dictionary (table definitions) and the User Edition.
- Discoverer Administrator - tool used to build and maintains the End User Layer.