Personal computing is browsing the web, playing music, watching movies, playing games, reading email, writing email, and creating documents. In short, it is everything I do for which a computer is the tool for getting done.
Smartphones, tablets, notebook computers, and desktop computers are all devices that I can use for personal computing; they all are personal computers.
Smartphones are about portability and have displays that are 6 inches or smaller. My smartphone is my most personal computer, I take it everywhere and it contains, or has access to, the information most important to me that I always need.
Tablets are about flexibility, and have displays that are between 7 and 12 inches. I could carry my tablet with me almost everywhere and it fits easily in my Waterfield Indie bag. Tablet displays are larger than smartphones so I can view more information, and small enough to be held and used while sitting or standing. Like a smartphone, a tablet can be used while only being held in your hands and does not require a surface.
Notebook and desktop computers are about large displays and inputting a high volume of text. Both require a surface to hold the device in order for it to be used. Between the two, I am unsure why one would buy a desktop rather than a notebook when the price is the same. Gaming is a niche that requires the highest performing desktop and notebook computers.
A person who needs to always type a lot of text, use large displays, or play games will not realize the value in the flexibility of tablets and therefore will see no value in them. Such people will point out that ultra-thin notebooks are as portable as tablets.
A person who values flexibility may prefer a tablet over a notebook computer. If they wish to, one can connect a keyboard to a tablet to input text in a growing number of adequate word processors. Many tablets can also be attached to large displays.
People who can afford to may own one or more of these devices, using each as a situation requires. At a minimum, almost everyone has a smartphone and adds to that additional devices as their budget allows.
Ten years ago everyone would start with a desktop or notebook computer and then add a smartphone, but today the order of priority has flipped, which is the big change in personal computing during this past decade. Consequently, authors of content and applications seeking the largest market for their work will target smartphones.