Oliver Queen spent five years on a hellish island after his father's boat, The Queen's Gambit, was destroyed. Being on that island changed him, and when he returned to Starling City, he became a vigilante using the skills he'd learned in his time away.
His original focus was to save his city, using the notebook his father gave him to punish those who had put it on the road to ruin. In time, and with additions to him team, his focus grew to protecting the city from any threat that he became aware of. The contact killer Deadshot, a sadistic serial killer called the Dollmaker, and many more who were trying to cause harm to the city and the people in it.
The Arrow wasn't always successful, though. Despite his best efforts, and getting so close to succeeding, Oliver failed to stop the Dark Archer -- family friend Malcolm Merlyn -- from using an earthquake device to destroy a huge chunk of the city. The subsequent earthquake killed hundreds, including Oliver's lifelong friend, Tommy Merlyn.
After the earthquake, Oliver left for a while. Eventually, he was tracked down by Diggle and Felicity, and persuaded to return to Starling City again. His family's company was in danger, the city was in danger, and copycats were calling themselves The Hood Gang, dressing like him and attacking the rich people on the city, stealing from them with no provocation.
When he returned, someone else came in to his life as well: Slade Wilson, someone that Oliver had thought dead after their last encounter on the island of Lian Yu. Wilson has a serious grudge with Oliver and is working to take away everyone and everything that Oliver loves, before killing him. Among other things, Slade has told Laurel about Oliver's secret, told his sister Thea about Malcolm being her biological father, caused the Queen family to lose their company and their fortune, set an army of men with the same enhanced physiology to destroy Startling City, and killed Oliver's mother in front of both him and Thea.
Tonight's episode promises to be quite the finale, with many storylines coming to a close. It should be really interesting to see if it can live up to its potential, and I'm really looking forward to the first commercial for the new Flash TV series! Buckle up, because tonight could be a hell of a ride!