The word tablet, of course, comes from the English, meaning blackboard, or notepad, and a tablet computer is a portable, very flat computer!
In short, it's a very flat computer you can work on in basic configuration without a keyboard or mouse! It is similar to a smartphone in operation and usually uses the same operating system (Android) and the same applications, called APPs!
Meanwhile, Microsoft has entered the market, this is called then Microsoft's tablet PC and Surface or Surface Pro and Appel is called the whole iPad!
To confuse everyone, there is still the Samsung Galaxy Tab !