iPads come in only two different colors: black and white. Otherwise, they all look pretty much the same. In fact, if you pick up an iPad, you're not likely to be able to tell one model from another. That's because the differences are under the hood. There are only two variations in iPad models: the amount of memory built into the iPad and how it connects to the Internet.
Deciding how much memory is enough
You can get three levels of memory. How much memory is enough for you? Here's a general rule: If you like lots of media, such as movies and photos or e-books, and you want to store them on your iPad, you probably need the iPad with the largest amount of memory. For most people who manage a reasonable number of photos, download some music, but watch heavy-duty media like movies online, a medium amount of memory is probably sufficient. If you pretty much want to check e-mail, browse the Web, and make short notes to yourself, the smallest-memory iPad might be enough.
If you bought the first version of iPad, you should know there is no way to expand memory. Memory resides on a micro-SIM card (a smaller version of the SIM card in your cellphone), which is fine for saving your contacts' addresses and similar data, but doesn't lend itself to video storage. Apple is banking on you wanting to stream and sync content. Only you can decide if that will work for you.
Determining if you need Wi-Fi only or Wi-Fi and 3G
You can buy an iPad with only Wi-Fi or with both Wi-Fi and 3G.
Wi-Fi is what you use to connect to a home network or your local coffee shop network. It's a network that has a reasonably limited range. If you leave home or walk out of the coffee shop, you can't get online.
3G is the cellphone technology that allows an iPad to connect to the Internet via a cellular network that is widespread, just as you can make calls from just about anywhere using your cellphone.
You can use a Verizon or Sprint MiFi router to take a cellular signal and change it into a virtual Hotspot, which means your Wi-Fi iPad could grab the signal as if it had 3G — and save yourself the cost of the data connection fee.
The 3G iPads are GPS devices, meaning that they know where you are and can act as a navigation system to get you from here to there. The Wi-Fi-only model uses a digital compass and a triangulation method for locating your current position, which is much less accurate.