Referred to as:

Bluetooth Low Energy

BLE

Bluetooth Smart

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+

iOS5+

Android 4.3+ (4.4+ recommended)

Device Types

'Central' – e.g. a mobile phone

'Peripheral' – a device which want to connect to a central device such as a phone

Device Discovery

Advertising Data

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.

Scan Response

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.

Broadcast Communications

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.

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