Using 'If' condition in formula field of SFDC

IF( ISBLANK( Close_Date__c ) , TODAY() - Open_Date__c, Close_Date__c - Open_Date__c )

- In the above formula field ISBLANK( Close_Date__c ) is the Condition.
- TODAY() - Open_Date__c will trigger if the condition is True.
- Close_Date__c - Open_Date__c will trigger if the condition is False.