Summarize this article:

Trapdoors can do the following:  Prevent falls. Prevent mob access to an area. Stop water, snowfall, rain or lava from flowing into an area. Act as an opening for a countertop, such as a bar. It still lets light come through and does not block redstone signals. This is the only way to place a trapdoor. Then you can build around it with other blocks. Right click on it. It will flip back onto the block it is attached to.

Summary:
Use trapdoors in your structures. To place a trapdoor, put on the side of a solid block. Open a trapdoor.