Cliff was the second oldest of the Tannen Brothers. In this alternate reality, their father Irving "Kid" Tannen never went to prison, allowing him and the Tannen family to reign supreme as a criminal syndicate for over fifty years in Hill Valley.
Behind the scenes
There was no mention of Biff's family, except for Gertrude Tannen and Griff Tannen, in the films. However, Back to the Future: Biff to the Future 1, co-written by Back to the Future co-creator Bob Gale, stated that Biff was Gertrude's only grandson. So, Cliff and Riff are the result of Kid Tannen not going to jail, and didn't exist in the timelines where he was imprisoned.
No real information about Cliff's personality are expressed, save for the Tannen family disposition towards meanness. He couldn't have been born any earlier than 1938, even if he was conceived directly after Biff's birth.
Cliff Tannen was voiced by Andrew Chaikin.