UDLD complements the Spanning Tree Protocol which is used to eliminate switching loops.
The goal of the UDLD protocol is to detect a broken bidirectional link (e.g. transmitted packets do not arrive at the receiver, or the fibers are connected to different ports).
UDLD is a Cisco-proprietary protocol but HP, Extreme Networks, and AVAYA all have a similar feature calling it by a different name.
Brocade/Ruckus Networks ICX Switches offer this feature as Uni-Directional Link Detection(UDLD).
Brocade devices running Ironware support a proprietary form of UDLD.