The Logical Link Control is a sublayer of the data link portion of the Open System Interconnection reference model (OSI Model). Defined in IEEE 802.2, LLC is responsible for the identification and encapsulation of upper-layer protocols in a layer-2 frame. The information provided by an LLC header contains instructions to the data link layer on what to do with a packet once it is recieved on a destination host (hand it over to the IP protocol on the network layer, for example).
LLC headers are only added to 802.3 Ethernet frames, as the Ethernet_II specification contained a built-in field designed to hold upper-layer protocol information. 802.2 frames can be identified by the source and destination SAP fields in their data link headers.