I would think that a flashback outlines specific events that happened in the past. Like the section with the children in Stephen King’s It. Where backstory is just exposition that fills in information about the characters past, without explicitly going in on it, like Jack’s alcohol abuse in The Shining.
A flashback is like what you see in movies. Characters are doing X and remember something. Scene changes (sometimes to greyscale) and shows the previous event. Backstory is someones background.
Flashbacks are a tool for telling the story, backstory is just story that happened before the main one you're telling. You might use a flashback to show the backstory, for example.
Flashbacks are glimpses into the past, may be recent or not. Backstory is when the damage is done, generally before the return journey begins along the cycle: https://www.youtube.com/user/clickokDOTcoDOTuk/videos
