String

String is an item primarily received by killing Spiders or Cave Spiders, which drop 0 - 2 strings on death, but can also be obtained by destroying Cobwebs and Tripwires, which drop only one piece. String has an approximately 53% chance of spawning in a Dungeon chest, where it will appear in stacks of 1 - 4 units. This is not a practical way to find String though, since killing spiders can be used to get string more quickly and in larger amounts. In Abandoned Mine Shafts and libraries in Strongholds there are Cobwebs which can be cut with a sword or shears to get string. Tripwire (along with Tripwire Hooks), generate in Jungle Temples, which can be punched for collection, but also sheared and not activate any redstone.

History
String was added to the game in Indev (February 19, 2010) as a drop from Spiders.

As of snapshot 12w22a, string can be placed on the ground as a tripwire. Tripwire was first revealed on Dinnerbone's Twitter. Dinnerbone released a video (below) showing him experimenting with Tripwire before its release.

As of snapshot 12w23a, tripwires is activated by all entities including arrows, and the max distance has been increased from 18 to 40 blocks.

Crafting
String is a vital component to two recipes, the Bow and the Fishing Rod. String can also be used to make Wool, but this is rather wasteful as Wool can be obtained from Sheep; it is more economical to save string for making more Fishing Rods or more Bows that can be used to make Dispensers.

String can be placed on the ground as a Tripwire. Hooking them up to Tripwire Hooks will allow Redstone current to be activated by mining or walking over the string. It is difficult to see the string, making it useful in traps.

String can also be obtained by cutting Cobwebs and Tripwires. Cobwebs are mostly found in abandoned mineshafts, but also in stronghold libraries. Tripwires are found naturally currently only in jungle temples.

Use as Tripwire
A Tripwire is a small, gray wire that is placed with String. Tripwire can output a redstone signal when walked over if configured correctly.

Usage
Tripwire can be activated to output a redstone signal, yet it must be attached to two Tripwire Hooks on each side to output the signal. Tripwire can be connected to other Tripwire for up to 40 blocks.Tripwire does not require a block under it to work. When a Tripwire connection is formed, the Hooks must be placed on both sides of the connection for it to have the ability to be activated. To activate a Tripwire when it is connected, an entity (mobs, minecarts, boats, arrows, items, etc.) must move on, through or over it. The redstone signal must connect to the Tripwire Hook to be activated, not the Tripwire itself.

Placing and removing Tripwires
Tripwires are placed with String. The actual Tripwire block can only be obtained through inventory editors or /give. This item can be placed like normal String can be placed. Tripwires connect like Redstone wire, and can connect to Tripwire on all four sides. Tripwire can be placed in the air, by placing it on top of any solid block. Tripwire can also be placed on other Tripwires. When breaking a block below Tripwire, the Tripwire is broken, activating the signal and String is dropped. If Tripwire is placed on Glass or Leaves, it will not connect to Tripwire Hooks or Tripwire placed on any block that is not Glass or Leaves. Yet, when the Leaves or Glass is broken, the Tripwire is not dropped nor activated (the Tripwire remains in the air). This the same for when one is broken that has another Tripwire placed on top. Tripwire has the same function when in the air as if it was on the ground. When making tripwires, there either needs to be no blocks under any wire/hook or all blocks under the wire/hook. When breaking Tripwire itself, it activates any Tripwire or Tripwire Hooks that it was connected to. To disarm Tripwire without activating the signal, you must break the block with Shears. This will remove the block and drop String, yet will not activate the signal. Tripwire has the hitbox of a Slab when it is not connected to Tripwire Hooks. When it is connected, the hitbox is the same as Redstone wire.

Trivia

 * Walking onto or off from any piece of string next to a BUD switch, will cause it to update even if the string isn't connected to tripwire hooks. Breaking the block with Shears will do the same.