Two common types of store security tags are RFID tags, which use a radio frequency, and ink tags, which burst and spray ink if a special tool is not used to remove them. These tags must be deactivated or removed by store personnel.
Very small RFID tags are concealed in store merchandise and are also often hidden in library books. Gates at the doorway of the establishment broadcast a continual radio signal between them. When a security tag that has not been deactivated passes through this radio signal, a tiny antenna or coil in the tag picks up the signal and sends back a slightly modified signal. When the gates receive this modified signal, they sound the alarm. The RFID tags are usually deactivated by passing them through a device at checkout.
Ink tags are simply small cartridges filled with ink that require a special tool to place them on objects and remove them. Attempts to remove them by force cause them to rupture, leaking ink all over the item. They are almost exclusively used with garments. They do not prevent a thief from leaving the store with the item but are intended to discourage someone from taking the garment due to the likelihood of ruining the garment while trying to remove the tag.