A tripwire is often used by the military to detect if the enemy is moving along a certain path. Sometimes this tripwire will be attached to explosive, some kind of flare or just a small bell placed near the people wishing to know.
There are a few techniques to detect tripwires but the easiest is for the front soldier to dangle a small wire vertically in front of him. If he gets near a tripwire he will feel the wire move backwards. The other soldiers can simply step over the tripwire and continue their patrol, after marking the grid reference for a later operation.
A web "tripwire" is used for the same purpose (minus the flare or explosive), it gets triggered when someone searches for a specific keyword and visits a website fitted with said "tripwire". The website will silently write a log entry including details like IP address, browser agent, date and time.
The data collected will be used by project: BackWatch as one of their tools for alerting their clients if/when someone is searching for them and in what context.