Purpose of Drag&Relate in Portal

Hi Experts,

I am started learning with Drag&Relate, I noticed help.sap links. I didn't get clearly the purpose of Drag&Relate.

What is the purpose of Drag&Relate?

What is the Difference Between Related Links and Drag&Relate?

And Which ServicePack need to work Drag&Relate in EP7.

I am working in EP7 SP9, it is not showing Drag&Relate Option..

It's very urgent, Please guide me regarding this.

Thanks & Regards,

Art.