A frame check sequence (FCS) refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction.
The sending host computes a checksum on the entire frame and appends this as a trailer to the data. The receiving host computes the checksum on the frame using the same algorithm, and compares it to the received FCS. This way it can detect whether any data was lost or altered in transit. It may then discard the data, and request retransmission of the faulty frame.
The FCS is often transmitted in such a way that the receiver can compute a running sum over the entire frame, including the trailing FCS, and expect to see a fixed result (such as zero) when it is correct. Such an FCS generally appears immediately before the frame-ending delimiter.
By far the most popular FCS algorithm is a cyclic redundancy check, used in the Ethernet, X.25, HDLC, Frame Relay, Point-to-Point Protocol (PPP) and other data link layer protocols.
Reference:
http://en.wikipedia.org/wiki/Frame_Check_Sequence
