Oracle Application Express (Oracle APEX), formerly called HTML DB, is a browser-based rapid web application development tool for the Oracle database. Using only a web browser and limited programming experience, you can develop and deploy professional applications that are both fast and secure. The programming language you need if the standard features don't fulfill your needs is PL/SQL.

Many existing Oracle developers are moving to APEX, because it is simple (no application server needed) and uses PL/SQL. There is a page on OTN comparing APEX to Oracle Forms.

Pro
There is no separate license cost for applications built with APEX – the product is a "no-cost" option with all database editions.

The wizards in the product make it possible to build simple, but robust, web applications very quickly.

Organizations with PL/SQL programming expertise will appreciate that the language for defining business logic beyond the built-in functionality is PL/SQL.

Because APEX applications run in the database, they perform very well (unless the SQL queries are written poorly by developers)– they do not (indeed, cannot) suffer from the excessive number of roundtrips between the application server and the database that characterize poorly performing three-tier applications.

Con
An Application Express application is subject to the normal web application limitations: A limited selection of user interface components. However, with built-in partial page refresh capabilities there is no need to submit and re-draw the whole screen for many operations.

If the APEX standard functionality does not meet your needs, you will have to learn the framework in order to add your own code at the relevant points – just like you had to learn the available trigger points in products like Oracle Forms. The APEX framework can be readily extended utilizing Javascript, AJAX, DHTML, etc. to incorporate Web 2.0 functionality.

Because APEX applications run in the database, including the business logic, you do not have the same performance requirements inherent with a three -tier architecture where you need to “scale out” (adding more application servers) to improve performance. The overall performance overhead on the database of running APEX is very small, allowing it to scale very well. For database intensive APEX applications (those with significant data processing) with large numbers of concurrent users it is recommended to utilize Oracle RAC to balance the database load. Multiple application servers can also be incorporated to ensure high availability.

More Info
Please visit the Application Express Home Page on OTN to access lots of great content.
If you have any questions go to the Oracle APEX OTN Forum.

The Application Express community is continually growing as can be seen by going to the Community and Partners Page where you can find consulting companies, hosting companies, Internet applications, commercial applications, blogs and customer links and articles.

The Application Express UnOfficial Wiki is also well worth checking out.