The first one to come up was Amazon Kindle but it was just meant for reading ebooks then came the Apple Ipad that took the whole commercial market with a storm, Apple Ipad's ability to not just be able to read any ebook but to be able to listen to music, browse the web chat, use Maps for navigation and recently they added Siri(a voice response app to speak and tell the ab do something for you instead of you doing the touch). Apple uses its own operating system iOS(the latest one today is iOS 6.0)
Same way many other companies like Sony, Motorola, Dell, Samsung, Micromax jumped in with Android Operating systems. and them being cheaper then Apple also had lots to offer. The cheapest tablet in India on Android would cost between INR 2000 to INR 5000.