What is the difference between a GATT Server and a GATT Client?

A GATT Server is a device which stores attribute data locally and provides data access methods to a remote GATT Client paired via BLE. A GATT Client is a device which accesses data on a remote GATT Server, paired via BLE, using read, write, notify, or indicate operations. Once two devices are paired each device can act as both a GATT Server and a GATT Client.