Glass Walls are transparent walls that can be used to make windows that let in natural light, while preventing monster spawns. They are crafted from Glass and also spawn naturally in houses on Floating Islands. They can be crafted into Stained Glass.
Placed Glass Walls can change to a different sprite upon exiting and re-entering a world.
All Stained Glass sell for 5 silver, which is a productive way to make money.
Prior to the 1.1 patch, Glass was only able to be placed like any other block, so it was not possible to place it in the background. Before Glass Walls were introduced, you could make holes in the background of your Home, but the size of those were limited due to housing requirements.