There are several theories of who Zoom actually is on The Flash. There is one theory that Zoom is Henry Allen. I kind of agree with that, but there is more to my theory. I believe The Flash (2014) exists in the same universe, or multiverse, as The Flash (1990); and that Zoom is Pollux. Pollux is from season 1 episode 18 "Twin Streaks" of The Flash (1990). He was an artificially created clone of Barry Allen "The Flash". He was given a suit very much like Flash's suit but it was blue, and when he ran he created a blue streak. Which is interesting because Zoom's lightning is blue.
At the end of Twin Streaks, SPOILER ALERT, Pollux dies then like the other attempted clones before him when he dies his body cells vibrate and keep vibrating, faster and faster until he phases out of existence. That's the best way I can describe it.
But I don't think he just phased out of existence. Although he was in the body of a middle aged man, he was just several days old. He had no idea of how much he couuld actually do with his super speed. His power was out of control in his final moments, and he actually phased through to Earth-2. There he recovered from his bullet wound, since he does have rapid cellular regeneration. But the just say his super speed affects his ability to heal as well. Anyways, he recovered from his wound, gets by through life on his own, and without someone to live with or to loveable grows crooked and twisted, while learning more about and how to control his abilities.
And one day he sees a red streak. He realizes there are even more speedsters in existence. He has had a terrible life and wants satisfaction. The only way he can think to do so is by becoming the fastest man alive.
And that brings us to season 2 of The Flash.