The tablet is part smartphone with thyroid problem minus the phone capability (for most tablets, at least), and part slimmed down notebook without a physical keyboard. It is less portable than the smartphone because it is typically too large to fit in your pocket, yet it is much more portable than netbooks or notebooks because it is slim and light--like carrying a hefty magazine. Most of the tablets today run a version of a mobile OS like a smartphone--iOS, Android, WebOS, etc.--so they may not be productivity workhorses. But, the tablet is capable of performing virtually all of the same tasks as netbooks or notebooks, while also doubling as a portable entertainment center with movies, music, and books.
Role: The tablet is a great mobile option for a variety of purposes. It can be used for Web surfing and email and limited productivity, as well as for entertainment and recreation. Its small size, light weight, and long battery life make it ideal for computing on the go.
The netbook is an attempt to take the portable computing experience of a notebook and make it even more portable. Netbooks are lighter than notebooks--typically weighing in at three pounds or less, and they have significantly longer battery life than most notebooks--six hours or more for most netbooks. However, the smaller size also comes with less CPU horsepower, less RAM, and less storage capacity than a notebook. To save space and weight, netbooks also generally lack a DVD or CD drive. The fact that the netbook runs a desktop OS (like Windows 7) means it can run the same software you use on your notebook or desktop PC, and qualifies it for more intense productivity. But, the small display and diminutive keyboard are handicaps that keep it from being a true workhorse.
Role: Netbooks are good when you truly need a physical keyboard and/or the ability to run the same applications as your desktop PC, but you want something lightweight and portable, with the battery life to survive a work day without recharging.
The notebook is essentially a desktop PC converted into a portable, self-contained unit. They vary greatly in size--ranging from 11-inch to 20-inch or greater display sizes--and they have the most horsepower of all of the mobile options when it comes to processors, memory, and data storage. Notebooks generally require separate luggage to cart them around in, along with a spare battery for when the juice runs out, and a backup power adapter for when the spare battery dies as well. Lugging a notebook around can be a workout--especially for notebooks on the larger end of the spectrum. The benefit, though, is that you are able to literally carry your desktop with you--sacrificing virtually nothing from the full desktop PC experience.
Role: The notebook is the mobile option of choice for true portable productivity. It is more a "portable" computing experience, than a "mobile" computing experience, but for tasks that require the full desktop OS, and the full physical keyboard, it gets the job done.
There you have it. There isn't a "right" answer. Even defining a "best" answer is a matter of subjective opinion. Don't waste your time trying to decide which mobile gadget is the mobile gadget--just realize that each serves a purpose, and choose the right mobile device for your needs at that moment.