Hi,

Script Includes are reusable server-side script logic that define a function or class. Script Includes execute their script logic only when explicitly called by other scripts. There are different types of Script Includes:

• On demand/classless
• Extend an existing class
• Define a new class

For more explanation you can refer the below link.

https://developer.servicenow.com/dev.do#!/learn/learning-plans/orlando/servicenow_application_develo...

Thanks,

Pooja M