Determinism means that something is predictable. Meaning, you can look at all the variables and predict what will happen in the future if you have enough data and know all the rules. It also means that if you rewind everything and set it up again, it'll do the same thing again.
So the movement of planets in the solar system is deterministic in that we can predict in advance what'll be happening.
The movement of balls on a pool table after the break should be deterministic because it follows simple rules, but tiny variations in initial positions get amplified so much so quickly that you don't know where the balls will go – you don't have accurate enough data to begin with. And if you re-set the balls, you can never get them exactly the same again and you'll get a different result. That's the definition of a chaotic system.. a bit like the weather systems in the atmosphere too.
Relating to people, we are sometimes deterministic, and sometimes not. For example, I bought my house shortly before the crash, which was perhaps a bad idea. But if you took me back to 2007, I'd have the same information to work with, the same values and experience, so I'd probly make the same decision again. However, some other decisions might be part of some random event in my brain that would be different if we repeated the whole thing.
If there is a god with free will, he might make a snap decision on something and alter the course of events… If however there's some kind of machine following simple laws, then given the same initial conditions the outcome woudl be the same.
