The most obvious difference between a tablet PC and a laptop is the lack of a keyboard. Tablet's rely solely on a touch interface on the screen for all input. This is fine when it involves mainly pointing, dragging or tapping to navigate around a program. The problems come in when you have to input text into a program when such as an email or document. Since they have no keyboard, users are required to type on virtual keyboards that have varying layouts and designs. Most people cannot type type as quickly or as accurately on a virtual keyboard. Users do have the option of adding an external Bluetooth keyboard to most tablets to make this more like a laptop but it adds costs and peripherals that must be taken with the tablet.
Result: Laptops for those that write a lot, tablets for those that do more point interaction.
This is probably the biggest reason to go with a tablet PC compared to a laptop. Tablets have the size roughly of a small pad of paper and a weight that is under two pounds. Most laptops are far larger and heavier. Even one of the smallest ultraportables, the Apple MacBook Air 11 weighs just over two pounds and has a profile that is larger than an iPad 2. The main reason for this is the keyboard and trackpad which require it to be larger. Add in more powerful components that require additional cooling and power and they get even larger. Because of this, it is much easier to carry around a tablet than a laptop especially if you happen to be traveling.
Tablets are design for efficiency because of the low power requirements of their hardware components. In fact, the majority of the interior of a tablet is taken up by the battery. In comparison, laptops use more powerful hardware. The battery component of the laptop is a far smaller percentage of the laptops internal components. Thus, even with the higher capacity battery of laptops, they do not run as long as a tablet. Many of the tablets right now can run up to ten hours of web usage before require a charge. The average laptop would only run for roughly three to four hours with a few systems able to stretch it out to eight hours but that still less than a tablet. This means that tablets can achieve all day usage which few laptops can achieve.
In order to keep their size and costs down, tablet PCs have had to rely on new solid state storage memory as a means to store programs and data. While these have the potential for faster access and low power usage, they have one major disadvantage in the amount of files they can store. Most tablets come with configurations that allow between 16 and 64 gigabytes of storage. By comparison, most laptops still use traditional hard drives that hold far much more. Extremely affordable netbooks still have 160 gigabytes of storage which allows for ten times the amount of data as the most affordable tablets. This won't always be the case though as some laptops have moved to solid state drives as well and may have as little as 64GB of space.
Since most tablet PCs are based on extremely low powered processors, they will generally fall behind a laptop when it comes to computing tasks. Of course, a lot of this will depend on how the tablet or laptop is being used. For tasks like email, web browsing, playing video or audio, both platforms will typically work just as well as neither requires much performance. Things get more complicated once you start doing more demanding tasks. For the most part, multitasking or graphics performance it typically better suited with a laptop but not always. Take for instance video editing. One would assume that this would always be in the laptops favor but a recent test of video editing on the iPad 2 with iMovie found its specialized video hardware actually provided higher performance than doing the same task with iMovie on a MacBook Pro. The difference is the