There are two HTTP gateway servers available in Oracle. One uses the Oracle listener to act as an HTTP server and it can deliver data from the database using PL/SQL stored procedures. The second involves using Apache and the Mod PL/SQL gateway. Each have advantages and disadvantages in using them.
The Apache HTTP server offers additional capabilities such as virtual directory configuration, SSL, URL rewrites, Single Sign On, and a whole range of open source plugins. The embedded gateway has limitations but is simple to configure and is designed to work with Mod PL/SQL (and Oracle Apex). It's lightweight and is embedded in the Oracle listener, requiring one less process to manage.