You can create an entire application by writing Deluge Script or create your application in Zoho Creator GUI and add form and field actions by writing Deluge Scripts.
Creating an entire application in Deluge Script
Refer the topic "Creating a New Application -> Deluge Script" in Zoho Creator Help Document, for more information.
Adding Form and Field Action Scripts Using Script Builder
When your form is created, you can add action scripts to your form or individual form fields using the Script Builder. The script builder can be invoked either from the Forms Tab or from the Script Tab.
From the Forms Tab
To add form action scripts when a new record is added, select the required form and click on More Actions -> Script (on Add). This will invoke the Script Builder in the Script tab.
To add form action scripts when an existing record is modified, select the required form and click on More Actions -> Script (on Edit). This will invoke the Script Builder in the Script tab
Refer Form Actions for more information and examples on writing form action scripts.
To add field action scripts when the value of a field in changed,
Select the form and move the mouse over the required field in the form.
Select Actions on Field Change. This will invoke the Script Builder in the Script tab.
To add action script whenever you update the value of a field in the form,
Select the form and move the mouse over the required field in the form.
Select Actions on Field Change. This will invoke the Script Builder in the Script tab
Refer Field Actions for more information and examples on writing field action scripts.
From the Script Tab
Selecting the Script Tab directly will invoke the Script Builder with the Form actions and Field actions displayed on the left-side tree.
Refer Script Builder for more information on using Script Builder to add scripts.