  The name "Ultra-Compact Dwarf Galaxy" (UCD) was invented for a new type of
astronomical object that has been discovered in cores of nearby galaxy clusters
a decade ago. UCDs resemble globular clusters, but are up to 100 times more
massive and slightly more extended. Their luminosities are comparable to those
of nuclei of the most massive dwarf ellipticals or late-type spirals. Various
formation scenarios have been brought forward to explain the origin and
evolution of UCDs. Two of them seem to be most promising: first, UCDs might be
the remnant nuclei of galaxies that have been disrupted in the cluster
environment. Second, UCDs might have formed from the agglomeration of many
young, massive star clusters that were created during ancient merger events. In
this contribution, I review the discovery history of UCDs, summarize their
observational properties in the fundamental plane of stellar systems and
discuss the implications for the different formation scenarios based on our
current knowledge about these enigmatic objects.
