The Appalachian Trail stretches across fourteen states, six units of the National Park Service, eight national forests, and sixty state parks and forests. The Appalachian Trail is mostly used by weekend or "short-term" hikers. The "thru-hikers" mainly start from the South in the spring and it takes them five to six months to complete the trail if they do so because it takes physical and mental strength to complete.
There are permits needed in Shenandoah National Park, Great Smoky Mountains Park, and Baxter State Park if you will be doing overnight camping other than that no permits and fees are needed. When hiking the trail three-sided shelters can be found every 10-12 miles, and there are a numerous amount of camp sites, and some of this camp sites require fees to stay there. If that is not for you, hostels and other places can be found in nearby towns. When hiking the Appalachian Trail food and supplies are important. There are stores in nearby towns, but most long-distance hikers prefer to resupply by having them sent to post offices on the trail. If you happen to get hurt while on the trail, hospitals can be found in the larger towns, but bring a first aid kit.
More information on the Appalachian Trail can be located at the following links:
An Appalachian Adventure
Potomac Appalachian Trail Club
The Appalachian Trail Conference
The National Park Service: Appalachian National Scenic Trail Homepage
Appalachian Trail in MD/VA