From Seo Wiki - Search Engine Optimization and Programming Languages
Web applications are popular due to the ubiquity of web browsers, and the convenience of using a web browser as a client, sometimes called a thin client. The ability to update and maintain web applications without distributing and installing software on potentially thousands of client computers is a key reason for their popularity, as is the inherent support for cross-platform compatibility. Common web applications include webmail, online retail sales, online auctions, wikis and many other functions.
In earlier types of client-server computing, each application had its own client program which served as its user interface and had to be separately installed on each user's personal computer. An upgrade to the server part of the application would typically require an upgrade to the clients installed on each user workstation, adding to the support cost and decreasing productivity.
Generally, each individual web page is delivered to the client as a static document, but the sequence of pages can provide an interactive experience, as user input is returned through web form elements embedded in the page markup. During the session, the web browser interprets and displays the pages, and acts as the universal client for any web application.
In 1996, Macromedia introduced Flash, a vector animation player that could be added to browsers as a plug-in to embed animations on the web pages. It allowed the use of a scripting language to program interactions on the client side with no need to communicate with the server.
Applications are usually broken into logical chunks called "tiers", where every tier is assigned a role. Traditional applications consist only of 1 tier, which resides on the client machine, but web applications lend themselves to a n-tiered approach by nature. Though many variations are possible, the most common structure is the three-tiered application. In its most common form, the three tiers are called presentation, application and storage, in this order. A web browser is the first tier (presentation), an engine using some dynamic Web content technology (such as ASP, ASP.NET, CGI, ColdFusion, JSP/Java, PHP, Perl, Python, Ruby on Rails or Struts2) is the middle tier (application logic), and a database is the third tier (storage). 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.
For more complex applications, a 3-tier solution may fall short, and you may need a n-tiered approach, where the greatest benefit is breaking the business logic, which resides on the application tier, into a more fine-grained model. Or adding an integration tier that separates the data tier from the rest of tiers by providing an easy-to-use interface to access the data. For example, you would access the client data by calling a "list_clients()" function instead of making a SQL query directly against the client table on the database. That allows you to replace the underlying database without changing the other tiers.
There are some who view a web application as a two-tier architecture. This can be a "smart" client that performs all the work and queries a "dumb" server, or a "dumb" client that relies on a "smart" server.  The client would handle the presentation tier, the server would have the database (storage tier), and the business logic (application tier) would be on one of them or on both.  While this increases the scalability of the applications and separates the display and the database, it still doesn't allow for true specialization of layers, so most applications will outgrow this model.
An emerging strategy for application software companies is to provide web access to software previously distributed as local applications. Depending on the type of application, it may require the development of an entirely different browser-based interface, or merely adapting an existing application to use different presentation technology. These programs allow the user to pay a monthly or yearly fee for use of a software application without having to install it on a local hard drive. A company which follows this strategy is known as an application service provider (ASP), and ASPs are currently receiving much attention in the software industry.
Writing web applications
There are many web application frameworks which facilitate rapid application development by allowing the programmer to define a high-level description of the program. In addition, there is potential for the development of applications on Internet operating systems, although currently there are not many viable platforms that fit this model.
The use of web application frameworks can often reduce the number of errors in a program, both by making the code simpler, and by allowing one team to concentrate just on the framework. In applications which are exposed to constant hacking attempts on the Internet, security-related problems caused by errors in the program are a big issue. Frameworks may also promote the use of best practices such as GET after POST.
Browser applications typically include simple office software (word processors, online spreadsheets, and presentation tools), with Google Docs being the most notable example, and can also include more advanced applications such as project management, computer-aided design, video editing and point-of-sale...
Browser applications typically require little or no disk space on the client, upgrade automatically with new features, integrate easily into other web procedures, such as email and searching. They also provide cross-platform compatibility (i.e., Windows, Mac, Linux, etc.) because they operate within a web browser window.
Standards compliance is an issue with any non-typical office document creator, which causes problems when file sharing and collaboration becomes critical. Also, browser applications rely on application files accessed on remote servers through the Internet. Therefore, when connection is interrupted, the application is no longer usable. Google Gears is a platform to ameliorate this issue and improve the usability of browser applications, it's use is being superseded by the introduction of HTML5 API's such as Web Workers and DOM Storage.
- Cascading style sheets (CSS)
- Content management framework
- Dynamic HTML (DHTML)
- Hypertext Markup Language (HTML)
- JavaServer Pages (JSP)
- List of web application frameworks
- Scalable Vector Graphics (SVG)
- Software as a service (SaaS)
- Software development
- WAR file
- Web 2.0
- Web services
- World Wide Web Consortium (Web standards)
- XRX (web application architecture)
- ↑ Alex Chaffee (2000-08-17). "What is a web application (or "webapp")?". http://www.jguru.com/faq/view.jsp?EID=129328. Retrieved 2008-07-27.
- ↑ James Duncan Davidson, Danny Coward (1999-12-17). Java Servlet Specification ("Specification") Version: 2.2 Final Release. Sun Microsystems. pp. 43–46. http://java.sun.com/products/servlet/download.html. Retrieved 2008-07-27.
- ↑ "Dynamic HTML and XML: The XMLHttpRequest Object". Apple Inc. http://developer.apple.com/internet/webcontent/xmlhttpreq.html. Retrieved 2008-06-25.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Jeremy Petersen. "Benefits of using the n-tiered approach for web applications". http://www.adobe.com/devnet/coldfusion/articles/ntier.html.