Advocates of Philosophical Free Will report that all people are deciding agents, who make decisions without interference from the universe at large, and that people are responsible for the implications of their decisions. If you decide to have ice cream, then this was entirely your decision, and the resulting pleasure (and weight gain) are both your own responsibility, and one cannot blame anyone else for chosing the way that you did.
Debates rage about how this fits in with the idea of Determinism, in which things have logical consequences and necessary requirements to have happened in the first place. Under hard determinism, everything that ever happened in the universe, and everything that will happen in the future, was pre-determined from the beginning and there is no other way that it could have happened. Some people argue that determinism is mostly soft, and it is compatible with the idea of free will, but others argue that determinism and free will are incompatible and that only one could possibly apply. These are further split by those that argue that there is free will and no determinism, and those that argue that there is perfect determinism and no free will.
I dislike the idea of determinism, at least in the absolute sense, because I think it undermines the idea of responsibility. If you promise not to get wildly drunk at the weekend party, and then screw up and get wildly drunk...well, then it was inevitable from the formation of the universe that it would happen that way. It seems to encourage a lackadaisical attitude, things are inevitable and don't really matter. Why try?
I also think that absolute free will results in equally silly ideas. We are not omnipotent and omniscient. Sometimes we make bad decisions because we don't have enough information to make a good one. Sometimes we have to follow sub-optimal things because we don't have the resources available to us to optimize. People are bullied into making bad choices, or tricked. Under pure free will, it's your fault when a bad product you made bankrupts the company, even if your boss ordered you to make it or be fired. This seems less than fair. Worrying about things out of your control just seems so...neurotic.
I know scientifically, that some things are deterministic. My computer, given the same instructions, will behave the same way every time. Engineering, chemistry, physics, and electronics relies on consistent reactions and would be impossible without them. And I would also like to believe that humans have at least some free will, and have some responsibility over things that they choose.
Free will also butts heads with religion. Deities, be they a pantheon of gods or one supreme one, are often described as being omniscient and knowing the future. However, if we humans have free will, then the future cannot be known until we make our decisions, as what we decide changes the future. If I decide to work in a research lab, then the future will be different than if I decided to work as a librarian instead. So if God or gods know about what I chose before I chose it, then was my choice really free? Knowing about what hasn't happened yet smacks of hard determinism. The best loophole I can think of is that gods or God know what I'm thinking now, know what the consequences of every choice are, and have some fuzzy-math confidence in which option I will take. (God is 73% confident that I will have a ham sandwich for lunch, and 82% confident that I will go to the bank today instead of Friday.) This is less than satisfying to theologins who proclaim absolute omniscience, and probably has a few loopholes in it, but it's the best I can think of for now for resolving the claims.
I would encourage my philosophical readers to write back with their own opinions on determinism versus free will.