What a fun collection! I had to google them to learn more and learned they have a shop called "Something's Hiding in Here" in Philadelphia. Here's a link to another photo of their houses. And here's where you can read more about them. I think they are pretty cool!
Ephemera is a broad term used to describe items of paper that were created for use in a short time span and meant to be thrown away after one or two uses. Items generally put into the ephemera category of collecting would be sheet music, posters, stock certificates, post cards, cigarette cards, magazines, catalogs, and the like.