Client scripts are shipped to the client (the browser) and run there instead of on the server. Users who know JavaScript can define scripts to run in the client browser. Several types of scripts are supported
Requirement : Whenever any drop down list value changed, Populate the selected value of Drop down list value into another field
Eg : Whenever 'Device type' value modified, display the selected 'Device type' value in 'Name' field
Solution
Create a client script as shown below screenshot
var deviceType = g_form.getValue('device_type');
var deviceTypeLabel=g_form.getOption('device_type',deviceType).text;
g_form.setValue('name',deviceTypeLabel);
For More Info , Refer http://wiki.servicenow.com/index.php?title=Client_Scripts#gsc.tab=0
