In Client side scripting technology, scripts are executed by the web browser on client machine while Server side scripts are executed on the web server before rendering the page to client's web browser.
Client side scripts can be embedded in HTML or these scripts can be kept in separate file (.js file in JavaScript) and referenced from the HTML page using it. When a URL is entered by user in address bar of web browser and the page contains associated client side scripts, then these scripts are downloaded from the server along with the page. Once these scripts are downloaded in browser memory of client these can be executed without any round trip the server. Client side scripts may contain instruction which gets executed on a particular event such as on text change of textbox.
Web developers usually write these scripts in JavaScript or VBScript. Client side scripts don't require any specific software on server what it requires is client Browser should support the scripting language. These scripts have more access to client's browser information than server side scripts. Client side scripts can be used to reduce overhead on server's limited resources if there is large audience.
In server side scripting scripts are executed on web server before sending the page to client's browser. Server side scripts require interpreter for the scripting language to be installed on web server. These scripts are used in cases where interaction to database or other resource on server is required. Commonly used server side scripting languages includes ASP, PHP, JSP, ASP.NET, Perl and many more.
Client side scripts behavior may be different depending on the web browser so web developers test these scripts on variety of browsers before hosting.