A hardrive or HDD is an older style of storage technology called a hard disc drive which has a spinning disc in side with an arm digitally writing saved information like movies pictures and documents onto the platter. This older form of technology is larger heavier and less durable and breaks easier due to the moving parts on the inside which bend or snap in an accident like dropping it.
But know there is a new hardrive called an SSD or a solid state drive it is called this as there are no moving parts hence the name SOLID STATE drive. This knew technology essentialy does the same thing except it is faster to access your files, it is smaller, lighter and more durable as there are no moving parts which means it would survive a fall. Most of the Apple products like Mac book air and knew ipad are so amazingly thin and light as they contain a SSD which allows them to be that thin and light. But the only down side of this knew form of hardrive is that it cannot contain as much information as an HDD.
This is inside an HDD
This is just one of the many products now using an SSD
This is an SSD that goes inside many of these knew product, like the Ipad