These are called Tablets and not Tablet PCs, just Tablets. PC, personal computer, has nothing to do with them.
I was going to say a similar thing, but even more curmudgeonly:
- A tablet PC is a laptop running Windows with a twistable, stylus operated touch screen: these typically weigh about 5+ pounds, cost well over $1000, and sell about 500,000 per year.
- The iPad (and its kin) is a device with a finger operated touch screen as its only built-in input device, running a lightweight "mobile" OS designed around touch input: these typically weigh about a pound, cost about $500 or less, and sell about a million per week. Apple has never called the iPad a tablet; instead its name harks back to its ancestor, the Newton Message Pad, and its intermediate ancestors are PDA's and touch screen phones, not tablet PC's.
- By the way, a tablet is a stylus operated auxiliary input device, mostly make by Wacom.
How the iPad and such got to be called "tablets", let alone "tablet PCs", based on one imperfect similarity (the touch screen, shared also with the iPhone and PDA's) is a mystery. Maybe Steve Balmer, Bill Gates, and Michael Dell wanted to pretend (as they all have) that the highly mobile all-touch iPad is just a slight variant on something that MS came up with a decade earlier. Usually, first movers and market leaders dictate naming, so it would make sense to call them all "pads", but "slate" might have more chance --- pity HP fumbled first use of that name.