At minimum, a BLE device such as your smartphone can be detected by a beacon when it comes in range, so people and devices can be tracked as they move through a space. You can think of it like indoor GPS. For this low-level use of beacons, any BLE device can be seen: iOS, Android, BlackBerry, Windows Phone, PC, Mac, and so on. In fact, it's not just computing devices; the technology could intelligently track hospital gurneys, golf carts, shopping carts, cars in parking lots, construction site tools, animals in a wild park, and so on.
The real power of beacons is the interaction they will enable via peer-to-peer communications when used with a smart device, which means that device's apps need to use the same APIs as the beacon and its back-end services do. That's where iBeacons APIs come in -- or similar APIs from Google, Microsoft, and other platform providers that decide to support the interactive BLE technology. The beacon makers also have their own APIs, of course.
When you go to a museum or store and get within range of an iBeacon, your iPhone could be asked to connect to a service that provides information, coupons, and so forth. Done ham-handedly, this could be a quick turnoff to people, who would perceive it as creepy spam. More likely, you'd need an app installed from which you opted in -- your museum might have an app, and certainly every major retailer will. Apple's been very pro-user when it comes to privacy, giving users control over whether their location is shared, microphone activated, and notifications enabled on a per-app and per-website basis; I'm not too worried about iBeacons becoming the mobile version of a walk down the Vegas Strip, at least not on Apple devices.
But it's not just about getting information from iBeacons related to what you are near. The technology also allows you to query the beacon and, thus, its back-end service, such as to find out where belts are in the store or when the next art film is showing in the museum -- and of course to buy related goods. In other words, iBeacons is a platform for interacting digitally with the real world, and that shoud be enormously powerful. The decade-old promise of location-based services should finally arrive. As an example, Major League Baseball today announced it will use the iBeacons technolgy in ballparks.
Also, the iBeacons API in iOS and OS X lets devices be their own beacons. That means an iOS device -- say, a retailer's iPod Touch-based checkout register or a firefighter's cellular iPad Mini -- could be the device that other devices interact with, becoming a mobile hub. For field forces and roving service employees, this too could be huge.