The historical development of mobile has gone from the laptop to the smartphone and, more recently, to the tablet. Laptops are "destination" computing systems. By that I mean that you take the laptop from one place to another where you can use it. Laptops can do everything, but in mobile the primary thing they do is "create" content. They are great for content creation because of the large screen, keyboard, Web browser, apps such as iTunes and Skype, and the OS and applications that support content creation from Microsoft Office (Word, PowerPoint, Excel and Outlook).
Smartphones are great for talking and messaging. You carry your smartphone around with you all day. You make a call. You check email. You do little "snippits" of things such as checking an email, checking postings to Facebook and sending a text message. The keyboard is typically on the screen (iPhone, some Android devices) but others have small keyboards that provide physical, tactile feedback. Smartphones are not meant to create a lot of content but, rather, to review what others have created and then make a quick comment or text.
Tablets, on the other hand, have the large screen like a laptop but are portable enough to carry around all day like a smartphone. They contain a mobile OS and user interface so the experience is more like a smartphone rather than a laptop. Tablets are great for reviewing and responding. It’s more comfortable typing on a keyboard that’s on a large 10-inch display. You can turn it sideways to more easily review Web pages and watch movies and TV shows.
Thus, in tablets, you can more easily review information and show it to others. Whereas laptops are often frowned upon for use in most upscale restaurants (tolerance is, however, growing), it’s perfectly acceptable and deemed cool to show someone something like photos or presentations using a tablet in such situations. Given these characteristics, each platform is excellent for different types of mobile tasks. In other words, mobile is a three-legged stool: One leg is the laptop, one is the smartphone and the third leg is the tablet. Together, they provide a stable platform for mobility.
In the end, the use of portable products is all about the user experience. Laptops are better for content creation, while tablets are better for reviewing and smartphones are great for talking and messaging. I believe that these three mobile device categories will be with us for a long time.