In addition to Andrew's answer I'd like to add a sample of SOQL using the TypeOf.

I faced the same issue time ago when I was trying to get different fields from Task's Parent depending of the parent type. In my case the parents were custom object.

Sample:

SELECT 
TYPEOF What
    WHEN ObjectA__c THEN fieldA__c, fieldB__c
    WHEN ObjectB__c THEN fieldC__c
    ELSE Name
END
FROM Task

This is other standard sample:

SELECT 
TYPEOF What
    WHEN Opportunity THEN Name, amount
    WHEN Account THEN Name, AnnualRevenue  
    ELSE Name
END
FROM Task

In addition to Andrew's answer I'd like to add a sample of SOQL using the TypeOf.

I faced the same issue time ago when I was trying to get different fields from Task's Parent depending of the parent type. In my case the parents were custom object.

Sample:

SELECT 
TYPEOF What
    WHEN ObjectA__c THEN fieldA__c, fieldB__c
    WHEN ObjectB__c THEN fieldC__c
    ELSE Name
END
FROM Task

This is other standard sample using Object.Type:

SELECT 
TYPEOF What
    WHEN Opportunity THEN Name, amount
    WHEN Account THEN Name, AnnualRevenue  
END
FROM Task
WHERE  
   What.Type IN ('Opportunity','Account')