Electronic Article Surveillance (EAS) is a technology used to identify items as they pass through a gated area. Typically this identification is used to alert someone of the unauthorized removal of items from a store, library, or data center.
There are several types of EAS systems and more information is here on those different technologies. In each case, the EAS tag or label is affixed to an item. The tag is then deactivated when the item is purchased (or legally borrowed) at the checkout desk. When the item is moved through the gates (usually at a door to the premises), the gate is able to sense if the tag is active or deactivated and sound an alarm if necessary.
EAS systems are used anywhere that there is a chance of theft from small items to large. By placing an EAS tag on an item, it is not necessary to hide the item behind locked doors and so makes it easier for the consumer to review the product.
Today's hot topic in EAS is source tagging, where the tag is built into the product at the point of manufacture or packaging. This makes the labelling of goods unnecessary, saving time and money at the store.