As with any tool, as a designer, it's always best to figure out what works best with you. In the terms of graphic tablets, the leader in the industry is the Wacom tablet, also pronounced many other different ways. They have different products, such as pen displays which are tablets that can range from this size, maybe 12 inches to about maybe 17, 22 inches, and you see the display on the actual tablet, which is really helpful if you're an illustrator or if you are a digital painter, or anything that really requires a lot of close up work, and that you want to work entirely on screen with.
Those are of course, the more expensive ones. Then there are the pen tablets. Those are the ones that you usually see off to the side, that a lot of designers use as well. I use them, not being an extreme illustrator, but it definitely helps with precision and with making sure that you're able to do edits where needed. It works best in Photoshop, any of the Adobe products, and then it also you can use it as a mouse too, as a natural mouse.
Another new thing that they've done, they've also created stuff for the mobile device. So they have an application for the iPad which is actually called Bamboo Paper. You can use it to also sketch down different ideas and notes.
As with any touchscreen device, you need a special tool to use the application, and this one that I have is the Boxwave Pen, which is a really good stylus. Most of them will always have a round tip, whereas actual pen tablets will have a more precise tip, a more chiseled point and you can actually change out the points as well.
With this you can just do what you would normally do on any type of pen, pencil or pen tablet, but you can actually see it on your device, and then when you're done, you can save it to photo, send it to Evernote, print or email. So it makes it really good to be able to sketch down different things along the way.
Pen tablets, pen applications, pen displays, they're all used for a lot of different things from sketching to extreme illustration but at the end of the day, based off your budget and based off what you're looking for, will determine what tablet works best for you.