More shelters are built along the AT every year. Eventually you will be able to hike from one shelter to the next without having to put up a tent. Till then, a tent is still a primary piece of gear you will need for your thru hike of the AT.
A good three season tent, like the ones you will see below are perfect for the AT. Consider the features you want in a tent. If you are hiking with a buddy, a tent with doors on both side of the tent make it easier to get in and out. A tent with less poles or clip construction will be faster and easier to set up than a tent with sleeves. If you are weight concious, you can go as light as a tarp that weighs ounces for your shelter. The key is finding what will be the best combination of features, weight, and confort. Take some time to set your tent up and use it a few times before you hit the trail.