Referred to as:
Bluetooth Low Energy
Part of the Bluetooth 4.0 specification.
Different specification to classic Bluetooth (it is a lightweight subset of classic Bluetooth).
Bluetooth Low Energy is now the best / simplest interface to include for connectivity to any modern mobile operating system (iOS, Android, Windows phone, etc). For iOS use no MFi program membership is required to use it (MFi licensing is required for classic Bluetooth).
Supported operating systems
Windows 8 (XP, Vista and Win7 only support Bluetooth 2.1)
Apple OS X 10.6+
Android 4.3+ (4.4+ recommended)
'Central' – e.g. a mobile phone
'Peripheral' – a device which want to connect to a central device such as a phone
Sent by a peripheral device at a frequency it decides (longer delay = better battery life). Received by any device which is listening. Can contain up to 31 bytes of data.
Can be optionally requested by a central device and allows a bit more data to be passed without an actual connection (e.g. device name etc). Can contain up to 31 bytes of data.
A peripheral can only be connected to one central device at a time and when connected it will stop advertising itself. Other devices can't connect to it until the existing connection is dropped.
A peripheral connects to a single central device, but actually a peripheral is able to broadcast a small amount of data within its advertising packet used for discovery which all listening devices can receive withtout actually connecting. For instance the Apple iBeacon concept uses this approach by inserting the 'Manufacturer Specific' Data field into the the advertising data.
Peripheral To Peripheral Connection
Data would need to be relayed via each connecting to a central device.