Client-side and Script-side Scripting

There are two main ways to customise Web pages and make them more interactive. The two are often used together because they do very different things.

Scripts

A script is a set of instructions. For Web pages they are instructions either to the Web browser (client-side scripting) or to the server (server-side scripting). These are explained more below.
Scripts provide change to a Web page. Think of some Web pages you have visited. Any page which changes each time you visit it (or during a visit) probably uses scripting.
All log on systems, some menus, almost all photograph slideshows and many other pages use scripts. Google uses scripts to fill in your search term for you, to place advertisements, to find the thing you are searching for and so on. Amazon uses scripting to list products and record what you have bought.

Client-side

The client is the system on which the Web browser is running. JavaScript is the main client-side scripting language for the Web. Client-side scripts are interpreted by the browser. The process with client-side scripting is:

1. the user requests a Web page from the server
2. the server finds the page and sends it to the user
3. the page is displayed on the browser with any scripts running during or after display

So client-side scripting is used to make Web pages change after they arrive at the browser. It is useful for making pages a bit more interesting and user-friendly. It can also provide useful gadgets such as calculators, clocks etc. but on the whole is used for appearance and interaction.
Client-side scripts rely on the user's computer. If that computer is slow they may run slowly. They may not run at all if the browser does not understand the scripting language. As they have to run on the user's system the code which makes up the script is there in the HTML for the user to look at (and copy or change).

Server-side

 

The server is where the Web page and other content lives. The server sends pages to the user/client on request. The process is:

1. the user requests a Web page from the server
2. the script in the page is interpreted by the server creating or changing the page content to suit the 
    user and the occasion and/or passing data around
3. the page in its final form is sent to the user and then cannot be changed using server-side scripting

The use of HTML forms or clever links allow data to be sent to the server and processed. The results may come back as a second Web page.

Server-side scripting tends to be used for allowing users to have individual accounts and providing data from databases. It allows a level of privacy, personalisation and provision of information that is very powerful. E-commerce, MMORPGs and social networking sites all rely heavily on server-side scripting.

PHP and ASP.net are the two main technologies for server-side scripting.
The script is interpreted by the server meaning that it will always work the same way. Server-side scripts are never seen by the user (so they can't copy your code). They run on the server and generate results which are sent to the user. Running all these scripts puts a lot of load onto a server but none on the user's system.