Desktops: A desktop computer is the most powerful computer you could own depending when you buy it and what your components are in the desktop. With that said if you are willing to pay your desktop computer can be one of the most powerful machines of all the classifications for computers. However the drawback means it is not mobile friendly. You do not want to pack this up and bring it to class or your workplace.
Laptops: These are potentially the second most powerful device you can get. Laptops are built to be on your lap. They are mobile friendly and you can move around with these devices without worrying about how hard it is to move. However, the disadvantage is you will never beat a well built Desktop in terms of processing power. Laptops are built smaller which do not allow the same in processing power.
Notebooks: Notebooks are even smaller than laptops and were one of the first devices to use the super fast SSD memory. This was because these devices were built to be even smaller than a laptop and have no cooling system (fans, moving parts, dvd drive etc) in them which means it has to back off on the processing power even more or else it would overheat. The advantage of these is they are very slim (more so than a laptop). However, as time goes on, the notebook and and laptop defintion line are being more and more blurred making it hard to distinguish a notebook and a laptop from each other. You may even see these terms used together.
Netbooks: These are similar to Notebooks, however, these devices are built to connect to the internet and function as a very cheap device that was only built for use with an browser. The easiest one to point to for an example is the Chromebook. Chromebooks are built to use the Google Browser and the extensions and apps from the Chrome Store only. It is not a very powerful processing device but it is a relatively cheap device and allows you to browse the internet.
Tablets: Tablets are quickly becoming hybrids of laptops and tablets. For example the new Microsoft Surface Book is a great example of a powerful laptop/tablet. Tablets are usually big handheld devices that use touch or a connected keyboard to function. Tablets can function without the keyboard. Tablets are quickly becoming replacements to Netbooks and Notebooks. Laptops are the next device up on the hybriding going on with Tablets.These will still not reach Desktop classification due to size limitations. Desktops will (for the foreseeable future) always be the most powerful option if you are willing to pay. Tablets are great for many things (watching videos, browsing the internet, creating documents, and etc). Tablets are very mobile.
SmartPhones: These are less powerful than tablets (again size matters and being smaller make it a similar situation with desktops and laptops). SmartPhones can do everything a tablet can but are usually not the easiest to work with (like typing documents on). These are very very mobile and can fit in most people’s pockets. These devices can call, text, and take great quality pictures (replacing most cameras that people usually would buy). These devices are continually getting better (like tablets) and are growing slightly in size to some being classified as a Phablet (hybrid of Tablets and Phones). These devices are the most common among everyone to browse the internet, and play game on. These are very handy devices to have.