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Mohd NabeelMohd Nabeel 

Why we need WhoId, WhatId and ParentId and others which i dont know

I know the definition of these Id's;. But i dont know what is the use of these ids and why we use it..Even with the accountId we can relate objects the why use these.Whoid is equivalent to Contact's ID or Lead Id then why we dont use those id's instead of who id or what id
Best Answer chosen by Mohd Nabeel
Devi ChandrikaDevi Chandrika (Salesforce Developers) 
Hi Mohd,
WhoId, WhatId are Polymorphic keys.If an object is to be related with multiple objects, then a separate relationship field has to be created for each relationship. Polymorphic keys are used to maintain relationship with multiple objects.Example Task which is related to account,contact etc.

Please refer below link which might help you

Hope this helps you
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Thanks and Regards