A Tripwire Hook, when activated with String, will send a redstone signal to whatever block it's on if an entity walks across it. They are excellent ways to make a detection system spanning a corridor due to their near-ivisibility; Pressure Plates are the only option for narrower areas.
The Tripwire Hooks must be placed on solid blocks straight across from each other, with no blocks between them, at most 40 blocks apart. String is then placed between them. Note that the String does not have to be on the ground; it can float.
Deactivating Tripwire Traps
Due to the fact that Jungle Temples contain riches guarded by tripwires, it's worthwhile knowing how to defuse them without getting shot. Here's what you need:
All you need to do is hit the tripwire with the shears. They are necessary because breaking a tripwire with anything else will send a short redstone pulse, which will result in an arrow being fired from a dispenser at you.