Determinism is the way things will be as a result of how things are and the work of natural laws. That is only to say that if we know exactly how things are at the present moment and the laws that govern how the world (or the universe) works, then we can derive how things will be at some future time. Bearing in mind that we've skirted over some problematic issues that we'll come back to, let's consider some examples.
Now consider something that many people hope eventually to achieve, namely that all the laws of physics, or of nature, have been discovered and understood, along with the (hypothetical) situation wherein we know the position and other characteristics of all fundamental particles (whatever they may be) in the universe. We can then apply the laws to (again) determine how everything that follows for these particles will play out over time. (Whether this is possible in light of certain other theories in physics and elsewhere is not important for the purposes of our example.)
Taking the first, what do we mean by "the way things are"? We can say that we are concerned with the state of the entire world, or universe, but why not take only a small portion instead? That would give us, say, determinism for a small region (or even particle) based on knowledge of how it will behave in future—a decidedly less ambitious endeavour. The problem here, though, is restricting the domain in this way: can we even do so at all? Although it may seem plausible initially, there are myriad factors that might influence the area we're looking at, especially if we're talking about laws of nature that are presumed to apply everywhere.
Free will
Clearly we've been concerned here with the extent to which a freely willed decision is really ours. It does no good to say that free will is what we have when we choose one direction instead of the other possibilities when beyond the influence of such circumstances, because situations like that are few and far between (indeed, we could argue that they don't exist at all). What we want to say, then, seems to be that insofar as we can, free will is when we are able to choose more than one option and do so by our own volition.
Consider now, though, not just external influences but those from "within". If I have a sweet tooth and choose candy instead of an apple, did I exercise my free will? What if someone who suffers from kleptomania steals something? (To hint at what will come, was it my fault, or theirs, respectively?) The issue here is whether such decisions can be called examples of free will.