There are approximately 250 shelters strategically spaced along the Appalachian Trail. The typical shelter is called a "lean-to", having three walls with one open side. They have a metal or shingled roof and a wood floor. All are marked on maps and trail guides, with most located near creeks or springs and some having a privy nearby.
Your guidebook will list locations and distances between shelters on the Appalachian Trail. The Appalachian Trail Conservancy publishes two highly recommended guidebooks, Appalachian Trail Data Book and Appalachian Trail Thru-Hikers' Companion. The Appalachian Trail Data Book is a condensed version of the most important information from other guides and is published every year. It includes the distance between major Appalachian Trail shelters. In comparison, the Appalachian Trail Thru-Hikers' Companion includes more detail. The latest version was published in 2010.
Shelters are intended for individual hikers and are a good place to stay dry in wet weather and talk with other hikers.
Staying in a shelter reduces hiker impact, though it is important to clean up after yourself as they can be an unpleasant place to stay when they are rodent infested. Another downside to shelters is that they fill up, especially in heavy-use areas during wet weather, so never completely rely on staying in a shelter.
Some shelters in the heavy-use areas require a permit, registration and/or a fee.
Published Mar 18, 2015. Appalachian Trail shelters close in Maryland, following the death of a hiker there on March 15th due to a falling tree. The safety closures are temporary.