Glass

Glass is a decorative block. Sunlight can penetrate through glass, making glass useful for buildings such as greenhouses. It has an almost completely transparent texture put on all six sides of the block. Glass blocks adjacent to other glass blocks are invisible when viewed through glass.

Glass will not hinder a redstone current from travelling down the side of a block diagonal to it.

Redstone (Wire), Jack-O-Lanterns, Minecart Tracks, Doors, Pressure Plates, Beds and Ladders cannot be placed on glass.

Glass blocks are crafted by smelting a block of sand.

As a Crafting Ingredient
As of Minecraft Beta 1.8, Glass Panes can be crafted by placing 6 Glass blocks in a rectangle.

As of Minecraft Beta 1.9 Pre-Release 3, Glass Bottles can be crafted with glass the same way as Buckets and Bowls.

Trivia

 * In Survival Test mode, glass blocks could be broken very quickly and would drop a glass block to pick up. The only way to obtain glass blocks at that time was to place them on the map before the level was loaded in the client.
 * Before Beta 1.6, when water was above glass, an infinite source could not be created.
 * When attempting to light glass on fire with the Flint and Steel, the durability of the tool is lowered without any flames appearing.
 * Placing a glass block on top of a chest does not prevent the chest from being opened.
 * Glass towers can create light underwater via sunlight.
 * Glass placed next to water running downwards will make the water run more quickly and violently; this can be useful in making traps.
 * When Glass is placed over a dropped item, the item is trapped inside the glass but can be retrieved normally.
 * Glass used to have a different texture when it was first created. Old Glass.png
 * Glass can be used for placing Redstone, ladders, torches or saplings on top on blocks that they can't normally be placed on like chests and furnaces by placing glass one block higher beside the chest/furnace and trying to place the Redstone, ladder, torches etc. on the side of the glass. This will place it onto the chest/furnace.
 * Before Beta 1.5_01, Rain used to fall through glass and other transparent blocks.
 * Mobs, except Spiders cannot draw line of sight through glass.
 * Narrow (1x1) towers of glass which mark Strongholds can be naturally found in worlds generated in Beta 1.9pre3. This is not a bug, but a debug feature which Jeb forgot to remove.
 * As of weekly snapshot 12w08a redstone torches and normal torches can be placed on top of glass, but not on the sides.