Laptops are basically scaled-down computers with a built-in screen and keyboard, with a battery to give them portability. All modern laptops have wi-fi.
Netbooks are very small versions of laptops, often with reduced functionality. They are, as the name suggests, primarily designed for accessing the internet whilst on the move. Nearly all have wi-fi.
Tablets are devices with a screen which can be anything from 4 inches (100mm) to 11 inches (280mm). They usually have a touch screen, and no keyboard. The most popular tablets are the Apple Ipad and the Samsung Galaxy. Most tablets have wi-fi, and some tablets have the ability to have a phone SIM inserted, which gives them access to the internet from just about anywhere. Tablets have varying levels of functionality, and you should search the internet for more information on this, if you feel that it is important to you.