The newest entry into this category are smartphones. They are small and very thin, thus ensuring great portability advantages, and can do most everyday tasks. Time has reduced the cost of smartphones, and it is possible to obtain one for as low as Rs. 8000. A smartphone is possibly the best investment one can make at this time.
Tablets only became common a few months ago, and are one of the best options for easily portable, powerful digital assistants. Their main advantage over smartphones is that they have larger screen which many will find to be a huge bonus. Additionally, tablets tend to pack much more punch than smartphones and can run more resource intensive applications. On the downside, it is less portable or practical due to it's size; a blessing and a curse!
Thin laptops including netbooks have been around in the market for a long while now. They have been around so much so that thinness is now the norm. They can run the Windows OSes to their full potential, while staying portable. Lugging around a laptop cannot be compared to carrying your mobile phone around, but for the sake of an argument, they are much less portable than either tablets and smartphones. They are (excluding netbooks) more capable than both tablets and smartphones, and can run very resource intensive applications for image and video editing for example.