This Topic is quite important and I recommend you to go through this thoroughly.
First I want to discuss terminology which you will encounter here & in most of Oracle Documentation.
Server - is a process or group of processes and provides a particular functionality/service For example, Database Server listen and process database requests, similarly Web Server listens for and processes HTTP requests.
Tier - is a logical grouping of services, may be on single machine or spread across more than one physical machine. For Example, Middle Tier in Oracle apps consist Form,Web,Report...Services. Forms & Report can be one machine 1 & Web Server can be on machine 2, still they will be part of same tier called as Application Tier or Middle Tier.
Node - is referred as Machine, Each tier may consist of one or more node and each node can potentially have more than one tier.
Desktop Tier - Oracle Applications/E-Business Suite is built on three tier Architecture with first Desktop Tier ic client machines accessing applications via browser(HTML based Self service applications) or Java Applet(Jinitiator) for Form based interface.
Application/Middle Tier - Second is Application Tier also called as middle tier consists of WebServer Forms, Reports, Concurrent processing, Discoverer and Admin Server. I suppose its known as Middle tier because it lies between our desktop & third tier called .
Database Tier - Third is Database Tier contain Orale database server which store all your application data. I am sure if you are DBA you know it :)
About Architecture of Individual Component like , Web Server, Form Server .... I will cover in my future posts.