My real estate book says that a backyard shed in the backyard of a residential house is an improvement. But it didn't say why. So if the shed(tuff shed) is not attached to the ground and is just sitting there, then why is it an improvement? If it is not attached then wouldn't it be personal property? It's just sitting on the ground.

A shed that is NOT on a concrete slab would be considered PERSOANL to the TAX guy. You will NOT get taxed for it. If it's on cement, then you will get taxed. They both serve the same purpose and are the same size. The base will make the difference on your taxes. The add value more when wired. Plus, you need a place to store your yard equipment.