The Peripheral is the device generating or serving the data. The Central is the device consuming these data. Starting with iOS 6, an iOS device can be both a Peripheral and a Central (never at the same time).
Each of the two components is represented by a class of the Core Bluetooth framework. The Central is represented by the CBCentralManager class, while the Peripheral is represented by the CBPeripheralManager one.
On the Central, a CBPeripheral object represents the corresponding Peripheral that the Central is connected to. Similarly, on the Peripheral, the CBCentral object represents the client the Peripheral connected to.
You can also think of the Peripheral as the device broadcasting the data. It starts advertising to the external world that it has some data. So, it advertises that it can provide services. On the other side, the Central starts to scan the environment looking for some service. If the Central finds the wanted service, then it asks the Peripheral to connect. Once the connection is established, the 2 devices start to exchange data.
Besides the Central and the Peripheral, we have to consider the structured data they exchange. These data are structured in Services. Each Service is then composed by different Characteristics. Characteristics are defined attribute types that contain a single logical value. If you go to http://developer.bluetooth.org, you can find the list of the standardized services and characteristics.
On the Central, services are represented by the CBService class. Each of the services is then composed by characteristics represented by the CBCharacteristic class. Similarly, the services and the characteristics on the Peripheral are represented by the CBMutableService class and the CBMutableCharacteristicclass, respectively. The following Fig. 2 highlights the structure of the classes as explained until now.