Oracle Application Express (abbreviated APEX, previously named Oracle HTML DB) is a web-based software development environment that runs on an Oracle database. It is fully supported and comes standard (at no additional cost) with all Oracle Database editions and, starting with Oracle 11g, is installed by default as part of the core database install.
APEX can be used to build complex web applications which can be used in most modern web browsers. The APEX development environment is also browser-based. [Source: Wikipedia]