Oracle Application Architecture R12

Oracle E-Business Suit (Oracle Applications) is a ERP Software the question is that What is ERP?

What is ERP:

ERP stands for Enterprise Resource Planning. A term that is used for business management systems which are designed to integrate the data sources and processes of an entire organization into a unified system. A key element is the use of a single database to store data for the various system modules like CRM, Finance, HR, Production, Sales etc.

Oracle Application R12 Architecture: 

Oracle Applications have a three-tire architecture. They are comprised of a Database Tier which manages the Oracle Database and stores all data, an Application Tier hosts various servers, manages communication between the desktop and database tier and contains the application file system and a Client Desktop through which user access Oracle Applications. 

A server (or services) is a process or group of processes that runs on a single machine and provides a particular functionality. For example, Web services process HTTP requests, and Forms services process requests for activities related to Oracle Forms. The Concurrent Processing server supports data-intensive programs that run in the background.

Note: 

A tier does not mean a physical machine. It's a logical grouping of services normally spread across more than one machine. A server is a process that runs on a single machine and provides a particular functionality. In Oracle Applications, one cab install the database and the application tier on a single machine and can start all servers from that machine.

Desktop Tier:

The client interface is provided through HTML for HTML-based applications, and via a Java applet in a Web browser for the traditional Forms-based applications. In Oracle Applications Release 12, each user logs in to Oracle Applications through the E-Business Suite Home Page on a desktop client web browser. The E-Business Suite Home Page provides a single point of access to HTML-based applications, Forms-based applications, and Business Intelligence applications.
Oracle JInitiator will no longer be required to run Oracle Forms in E-Business Suite Release 12. Oracle Forms in Release 12 will run directly in the native Sun Java2 Standard Edition plug-in.
The Forms client applet is a general-purpose presentation applet that supports all Oracle Applications Forms-based products, including those with customizations and extensions. The Forms client applet is packaged as a collection of Java Archive (JAR) files. The JAR files contain all Java classes required to run the presentation layer of Oracle Applications forms.

Database Tier:

The Database tier contains the Oracle Database Server which store all the data needed by Oracle Applications. The database stores the Oracle Applications online help as well. The database tier contains the Oracle data server files and Oracle Applications database executables that physically store the tables, indexes, and other database objects for your system.The database server communicates with middle tier and does not communicate directly with the desktop tier.

          For high availability environments, real application cluster can also be configured with Oracle Applications. In this case, more than one instance of Oracle is run. The data files are stored at a central location accessible from all of the instances.

Application Tier:

The application tier has a dual role: hosting the various servers and service groups that process the business logic, and managing communication between the desktop tier and the database tier. This tier is sometimes referred to as the middle tier.

Three servers or service groups comprise the basic application tier for Oracle E-Business Suite:

Web services
Forms services
Concurrent Processing server
In Release 12, Web and Forms services are provided by Oracle Application Server (OracleAS) 10g. They are no longer servers in the sense of being a single process, as was the case in previous Applications releases.

Note: There is no concept of an Administration server in Release 12. By default, patching can be undertaken from any application tier node.

Web Services: 

The Web services component of Oracle Application Server processes requests received over the network from the desktop clients, and includes the following components:

Web Listener (Oracle HTTP Server powered by Apache)
Java Servlet Engine (OC4J)
Oracle Process Manager (OPMN)
The Web listener component of the Oracle HTTP server accepts incoming HTTP requests (for particular URLs) from client browsers, and routes the requests to the appropriate OC4J container.
If possible, the Web server services the requests itself, for example by returning the HTML to construct a simple Web page. If the page referenced by the URL needs advanced processing, the listener passes the request on to the servlet engine, which contacts the database server as needed.

Forms Services: 

Forms services in Oracle Applications Release 12 are provided by the Forms listener servlet or Form Socket mode, which facilitates the use of firewalls, load balancing, proxies, and other networking options.
The Forms listener servlet is a Java servlet that delivers the ability to run Oracle Forms applications over HTTP or HTTPS connections. It hosts the Oracle E-Business Suite forms and associated runtime engine, mediating the communication between the desktop client and the Oracle database server, displaying client screens, and initiating changes in the database according to user actions.
The Forms listener servlet caches data and provides it to the client as needed, for example when scrolling through multiple order lines that exceed the limitations of a single screen.

Concurrent Processing server:

server which run reporting programs and data updating programs (periodically or ad hoc). These programs (named concurrent programs) run on this server in background in order not to interfere with the normal operations. When you submit such a request, either through HTML-based or Forms-based Application a request is made, a row is inserted into FND_CONCURRENT_REQUEST table. The concurrent managers read these requests in the table and start the relevant concurrent programs;

See also:
Step by Step Oracle Application R12 Installation with screen shots
Oracle Applications R12 imprtant/log files locations
Conucurrent Manager 11i
Patching
adadmin
Defining Custom Concurrent Managers