Biff Tannen was the school bully at Hill Valley High in 1955, and despite his low intelligence got by gradewise by picking on George McFly and making him do his homework for him. In the original timeline, George never stood up to Biff and so Biff continued bullying him all his life well into 1985 and became his boss at work as well. However when George's son Marty McFly travelled back in time to 1955, he changed history so that George finally punched Biff in the face during a confrontation at the Enchantment Under the Sea Dance, and from then on Biff remained extremely cowardly and subservient around George and his family.
In 2015, a 78-year-old Biff even allowed himself to be bullied by his own grandson, Griff. However after observing Marty McFly and Doc, he realized that the scientist had invented a time machine using a DeLorean. Taking the discarded Gray's Sports Almanac, Biff stole the DeLorean while Marty and Doc were attempting to prevent Jennifer Parker from encountering her future self, and went back in time to 1955 where he gave the almanac to his teenaged self, warning him to be prepared in case "a kid and a crazy, wild-eyed scientist" (Marty and Doc) came asking about the book. He then returned to 2015 where he suffered from a heart attack from the stress of the journey and died (in the original script, Biff actually faded out of existence because by giving his younger self the almanac he set in motion a chain of events where Lorraine McFly shoots and kills him in 1996, because of the events described below).
Using the almanac to bet on sporting events, young Biff made his first million at age 21. His winning streak earned him the nickname "the Luckiest Man on Earth," and after starting his own corporation in the 1970s, BiffCo, Biff became one of the most powerful men in the United States. His influence was so far-reaching that he even had a hand in helping Richard Nixon get re-elected as President of the United States. After a string of failed marriages, Biff, frustrated at not being able to have Lorraine McFly and made even more volatile and cruel than normal because of his wealth and power, killed George McFly and married his widow, becoming stepfather to Marty, Dave and Linda. He soon opened a casino in Hill Valley, Biff Tannen's Pleasure Paradise, by 1985.
All this was undone when Marty was able to go back in time to 1955, steal the sports almanac from the younger Biff after his older self gave it to him, and burn it. The normal timeline was restored, and when Marty returned to 1985 after a detour to 1885 and en encounter with Biff's ancestor "Mad Dog" Tannen, he discovered that Biff was the way he was before: shy and timid following George finally standing up to him at the Enchantment Under the Sea Dance.