What is Oracle Application Express?

Oracle Application Express or APEX as it is famously known is a rapid application development for the web closely coupled with Oracle's flagship product, its database. It was earlier known as HTML DB.

It's key features are:
Reporting
Forms
Charting
PDF Printing
Web Services
Spreadsheet Upload
Session State Management
User Interface Themes
Flow Control & Navigation
Conditionality on all Components
External Interfaces & Extensibility
Declarative BLOB Support
Optional Runtime-Only Installation
Security
SQL Workshop Tools
Supporting Objects Utility
Performance
Packaged Applications
Hosted Development


For more information read through What is Apex