The question is older than philosophy: when you decide between coffee and tea, is the decision genuinely yours, or just what your brain was always going to do? Compatibilists say "yours" can mean "made by your reasoning, given the circumstances" — that's enough for free will, even if the universe is deterministic. Hard determinists say no — every decision is the inevitable output of prior causes, and our sense of freedom is an illusion.
There's a third camp: libertarians (in the philosophical sense, not the political one) think genuine alternative possibilities exist — that the future is genuinely open, and we genuinely choose. Modern science complicates this: quantum mechanics offers randomness but not obviously agency, and neuroscience experiments (Libet, Soon, Schultze-Kraft) suggest your brain commits to a decision before you become consciously aware of it.
What's at stake: if there's no free will, do moral responsibility and praise and blame still make sense? Most philosophers think yes, in some form — but the answer reshapes how you think about punishment, addiction, and self-improvement.