Redstone Wire

Redstone Wire is a material used to transfer redstone charge between blocks. Charges can be used to activate certain blocks, such as doors. It is the placed form of redstone, which can be found by mining redstone ore with an iron pickaxe or better. Unpowered wire will appear dark red, and powered wire will appear bright red with animated sparkling.

Usage
Redstone wire will transfer power provided by power sources from block to block. It can be connected to other wires to create circuits. However, it will not try to form visible connections with blocks or items that "receive" power, like doors. These blocks need a straight wire run to or onto them in order to receive power. Redstone wire cannot be placed on glass or glass panes.

It is possible to tell whether a wire is connected to other blocks by its appearance: when placed alone and unconnected it looks like a dot on the surface of the block, but it will power all four surrounding blocks. If it is connected in all four directions, a wire looks like a cross (+). If it has one connection or two on opposite sides, it will look like a straight line (- or |), depending on the location of the blocks connected). Bends and T-junctions are also possible, when connected on the appropriate sides.

Powered redstone wire will glow dimmer the farther it is from its source of power. Power will only travel through 15 blocks of wire, beyond which it stops sparkling entirely. It is possible, however, to extend this range with gates or repeaters. The creation and use of gates are explained in the redstone circuits article.

Redstone wire will connect by running straight up and down the sides of blocks when changing levels, but there are some restrictions: the wire will only cross a 1-block difference in height. Also, the connection will be broken if an opaque block is over the lower wire. (Visually, the block is cutting off the connection with its lower edge.)

Trivia

 * If you were to hack your inventory to give you redstone wire (not the item, the actual block), it will look gray in the inventory. This is because it is gray in the texture so it can be colored according to its power level. Also, it is called "Redstone Dust" while the real item form is called "Redstone".
 * Redstone wire has a light level of 1 when not powered.