Well here are some pictures of 2 different shelters along the Appalachian Trail. These are spaced throughout the trail to where they avg about one every 9 miles. They are basically 3 sided structures ... either made of stone or wood, depending on which state you are in. They usually have a tin roof, which you hope doesnt leak! Some of the more upscale ones have 2 stories, picnic tables and even a fireplace!
The only bad thing about shelters are that the animals have learned... where there are humans....there usually is food! Which means, shelter usually have mice infestations. Then that leads to snakes coming in to dine on the mice...which is good thing because the mice tend to chew through your pack and destroy your gear. Some hikers have reported mice running over their head while sleeping! Now that is a way to be jostled out of a restful sleep!
Also you have spiders to deal with (which Amee hates with a vengence...even thought Amee is a million times larger than they are!). Then if you are real lucky...usually in the smokey mountains...you could get a BEAR to visit you and your food sack (which you really need to hang in a tree!). I really want to see a bear and a moose!
Usually the shelter has a water source near by whether it be a mountain spring or running brook. Also they usually have a privy (basically an outhouse!) to do your business...and if your really lucky...they will have TP in them! This definetly beats digging a hole out in the woods! The funny thing is that there is a privy that i read about in a journal that doesnt have any walls....because the 360 degree views are too breathtaking not to see! I am anxious to see that one...hopefully not having to use it!
These shelters are just another option to stay in for the night if the weather is too severe to sleep in my tarp, and they usually house anywhere from 4 to 20 hikers at a time. The thing is that not only do thru-hikers use them but also overnight hikers, boy scouts, and the occasional drunken teenager get togethers! So they can be very noisy and busy!