Are @2x images simply larger in terms of height and width or do they actually have a higher PPI (pixel density)?

Are @2x images simply larger in terms of height and width or do they actually have a higher PPI (pixel density)? If they are truly higher PPI, in terms of workflow of creating responsive images (and particulary retina) I've been told by trainers that it is important to link to or embed a higher PPI quality photo as a smart object -or- true vector (use vector shapes for icons, etc.) within your Photoshop document so that when you are ready to extract the images @2x, Photoshop will be able to generate (i.e. upsize to 2x) a crisp, high quality photo file. Is this aim correct or am I over-complicating retina images? I'm creating a responsive website design for desktop, tablet, and mobile, trying to figure out workflow. thx