Profile, Services and Characteristics

GATT transactions in BLE are based on high-level, nested objects called Profiles, Services and Characteristics, which can be seen in the illustration below:

Profiles

To use Bluetooth wireless technology, a device must be able to interpret certain Bluetooth profiles. Bluetooth profiles are definitions of possible applications and specify general behaviors that Bluetooth enabled devices use to communicate with other Bluetooth devices.

A Profile doesn't actually exist on the BLE peripheral itself, it's simple a pre-defined collection of Services that has been compiled by either the Bluetooth SIG or by the peripheral designers.

Services

Services are used to break data up into logic entities, and contain specific chunks of data called Characteristics. A service can have one or more characteristics, and each service distinguishes itself from other services by means of a unique numeric ID called a UUID, which can be either 16-bit (for officially adopted BLE Services) or 128-bit (for custom services).


Characteristics
 
The lowest level concept in GATT transactions is the Characteristic, which encapsulates a single data point (though it may contain an array of related data, such as X/Y/Z values from a 3-axis accelerometer, etc.). Similarly to Services, each Characteristic distinguishes itself via a pre-defined 16-bit or 128-bit UUID.

Characteristics are the main point that you will interact with your BLE peripheral, so it's important to understand the concept. They are also used to send data back to the BLE peripheral, since you are also able to write to characteristic. You could implement a simple UART-type interface with a custom 'UART Service' and two characteristics, one for the TX channel and one for the RX channel, where one characteristic might be configured as read only and the other would have write privileges.