What can glass be used for?
Use the following link for the different ways to use glass.
http://terraria.gamepedia.com/Glass
The gamepdia for Terraria pretty much covers everything about crafting so if you have any questions of "how to craft __" or "what can I use __ for" I would check there first for the answer.
Glass can be used for a number of things it can
- Be placed as a block
- made into backgorund wall
- crafted into a number of glass items like "bottles" to hold water
When glass is placed as a block it lets light in while keeping mobs out