Web and Client-server Structure

Web Application Structure
Web application is commonly structured as a three-tiered application.
Web based architecture is multi-tier involving atleast a database server(third tier), an application server(middle tier-Dynamic Web content Technlogy Ex:ASP, ASP.NET, CGI, ColdFusion, JSP/Java, PHP,embPerl, Python) and a frontend server(First tier-Web Browser ex:Internet Explorer,Mozilla,Netscspe etc).
Though many variations are possible, a Web application is commonly structured as a three-tiered application. In its most common form, a Web browser is the first tier, an engine using some dynamic Web content technology (such as ASP, ASP.NET, CGI, ColdFusion, JSP/Java, PHP,embPerl, Python, or Ruby on Rails) is the middle tier, and a database is the third tier. The Web browser sends requests to the middle tier, which services them by making queries and updates against the database and generates a user interface.

Client-Server Application Structure
Client-server architecture is two-tier.
The client-server software architecture model, distinguishes client systems from server systems, which communicate over a computer network. A client-server application is a distributed system that constitutes of both client and server software. A client software or process may initiate a communication session, while the server waits for a requests from a client.