Frame Relay is based on X.25 with some changes. Essentialy, With the introduction of quality digital phone services and error correction in modems, the overhead of X.25 was no longer needed. You can say that Frame Relay is X.25 with the error correction systems removed.
The following appeared in Wikipedia:
"Frame relay has its technical base in the older X.25 packet-switching technology, designed for transmitting analog data such as voice conversations. Unlike X.25, whose designers expected analog signals, frame relay offers a fast packet technology, which means that the protocol does not attempt to correct errors. When a frame relay network detects an error in a frame, it simply drops that frame. The end points have the responsibility for detecting and retransmitting dropped frames. (However, digital networks offer an incidence of error extraordinarily small relative to that of analog networks.)"
There is more to the topic than this, check:
en.wikipedia.org/wiki/X.25
en.wikipedia.org/wiki/Frame_relay
