Microsoft has just released their new operating system, Windows 8. To really simply things, I would say that Microsoft feels they are losing to Apple and Google and they need a tablet operating system and an App Store. By most accounts, Windows 8 is a good tablet (i.e. touch screen) operating system. Also, by most accounts, people who like Windows 7 on their desktop or laptop will NOT like Windows 8.
My recommendation is that you only buy Windows 7 for your non-touch screen laptop or desktop computer. Wait until the market has had time to evaluate Windows 8 before you jump on the bandwagon.
Do not confuse Windows 8 with Window RT. Microsoft says that Windows RT is a version of Windows 8. True, I suppose. But Windows 8 will run your old Windows 7 applications and Windows RT will not. Windows RT will only run applications purchased through the Windows app store (known as Windows Store). You will primarily find Windows RT on mobile devices like phones or tablets.
My recommendation is that you wait before purchasing a Windows RT device. Windows RT is new and untried and the Windows Store has far, far fewer apps that either the Apple or Google app stores. It is very unusual when any market has 3 strong competitors. Here we have Apple, Google and Microsoft all competing. Apple and Google are well established. Microsoft, in my opinion, has an uphill battle.
There are a number of tablet-like devices that run Windows 7 and Windows 8. The market refers to most of these devices as slate computers. The existing devices are rather large and heavy and have poor battery life compared to tablets. Admittedly, the line between these tablet and slate is blurry. My recommendation is to stay away from slate computers, especially the ones running Windows 7. I believe these devices will be obsolete within months.
Microsoft has released their own tablet called Surface. Initially, it will only be available with Windows RT. In early 2013, a version supporting Windows 8 will be released. It will be known as Surface Pro. It is still too early to know whether the Surface will be a success or not. One current downside is the limited number of applications in the app store.
Google licenses a version of its Android operating system for use in tablets. Several hardware vendors supply tablets based on the operating system. Google also has its own tablet, the Nexus. The app store is similar to Apple's but not as large. Generally the Android tablets are cheaper but, I believe, of a little poorer quality than the iPad.
Apple's iPad established the tablet market. Apple's devices are well engineered and proven. The app store has more apps than you can count. Apple's devices will keep their value but they are also relatively expensive.
The Amazon Kindle Fire is a surprising alternative. It is cheap and well received. Battery life is outstanding. It is more than a book reader but not as functional as a full tablet.
It is hard to make a recommendation here. Today, you can't go wrong with an Apple device but you will pay a premium for them. Apple likes to say their devices "just work". This is because Apple's devices are closed. Apple controls everything. Unlike the closed Apple devices, Google's strategy is to be open. Theoretically, Google's strategy will lead to innovation and cost competition. Or, it may lead to a proliferation of problematic and incompatible devices. Only time will tell whose strategy is superior.
I will say that today I own an iPad tablet and an Android phone. My guess is that my next tablet will be Android based. I am strongly considering a Kindle Fire for casual book reading and browsing. Still, my advice on the matter of which tablet to buy is worth exactly what you paid for it.