Hi Raj,
Business Rules
A business rule is a server-side script that runs when records are displayed, inserted, updated, deleted, or queried. Business rules are set to run at specific times during the manipulation of a form in a ServiceNow instance.
Client Script
A client script is a piece of JavaScript code that runs on the client, rather than the server. Well-designed client scripts can reduce the amount of time it takes to complete a form. However, improperly implemented client scripts can significantly slow down form load times. With the exception of the CellEdit client script, client scripts apply to forms only
Important point to be noted:
There is a lightweight GlideRecord object on the client side that can be used to query the server from within client scripts. For details, see Client-side GlideRecord. The following are some common methods that are available.
In addition to the link Harish mentioned. you can have a look on the below mentioned links .
Business Rules
https://hi.service-now.com/kb_view.do?sysparm_article=KB0540192
Client Scripts
http://wiki.servicenow.com/index.php?title=Client_Script_Best_Practices#gsc.tab=0
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