It is a Business Intelligence Reporting Tool from Oracle, you could run ad-hoq queries produce an output.
present it in a proper report format, as required by the business.

Administration Edition  : Used to create End user Layer which hides the complexity of the database.
Discoverer Plus  : Enables us to query a database and save the output in workbook analyze it and format it as per business requirement
Discoverer Viewer  : Enables us to view the workbook created in Discover plus using web-browser

Discoverer Administrator funcitons are

Gather the requirement of end user
Creates end user layer
Creates Business Area
Load Data in to Business Area
Creates Joins and Conditions
Send report to users.

Business Area : is a Logical Grouping of tables and views.
It is a collection of folders. A folder is a table loaded from the database.
Folder is collection of items, each item represent a column in a table.

Discoverer Desktop : 
Discoverer Viewer
Discoverer Plus :