You probably don't know who Ur was, and even if you do, you don't know much about him because there's not much to know. We know the following - Ur was a mage who lived in Dalaran before the coming of the Scourge to the Eastern Kingdoms, who apparently died during the invasion Arthas led to retrieve the Book of Medivh so that Kel'Thuzad could use it to summon Archimonde the Defiler. It's possible that Ur, being a remarkably skilled and knowledgeable wizard, was one of the three archmages slain by Arthas while maintaining auras that protected Dalaran from the undead.
What's interesting about Ur isn't what he himself actually did, however, but what was done with the work he left behind. Ur was the author of two tomes (that we know of), The Book of Ur and Ur's Treatise on Shadow Magic. Ur's knowledge of other planes of existence was significant, if faulty - while in practical terms his understanding was great enough that it was possible for Archmage Arugal of Silverpine to use the Book of Ur to summon worgen forth from their prison in the Emerald Dream, it's notable that Ur didn't actually understand what the worgen really were, or what druid magic was - he simply saw the worgen as monsters from another world. Still, without Ur, it is unlikely that Arugal would have been able to bring forth the worgen he did.
Amazingly, despite knowing nothing of druidism and only having a hazy understanding of the worgen origin Ur's research allowed Arugal to pull Alpha Prime and other ancient members of the Druids of the Scythe to Azeroth. Considering it was Archdruid Malfurion Stormrage who placed Ralaar Fangfire and the other druids who'd helped create the Scythe of Elune under Daral'nir (the great tree in the Emerald Dream for which Tal'Doren is a mirror) in the first place, the fact that Arugal managed to draw them forth is astonishing. Yet manage it he did, with Ur's book.