Determinism vs Free will
Determinism is the idea that for a given system, we could theoretically predict the outcome of phenomena if we knew the exact state and properties of each components. This idea can however only partially describe our reality. In a world of pure determinism, there would be no such thing as 'free will'. That is to say: we would be robots. Some people believe in this idea, some people don't.
We sometimes believe living creatures to be nothing but robots dominated by the laws of nature (determinism). In a world of pure determinism, everything must be symmetrical since these laws are the same everywhere.
In reality, it is only partially the case. Things in general may be similar, but the world of forms is very diversified, for instance, two stars or two flowers of the same species may look superficially identical but are in fact very different from each others.
As part of a bigger system, we are still heavily influenced by other components and this is why we sometimes believe in absolute determinism. Free will is not an illusion. The choices we make shape the path we take while we move in the tree shaped time. As long as there is consciousness, there is free will, and consciousness is everywhere.
See: quantum world
Since time is not linear, for each state of a system, there is more than one absolute possible outcome. That uncertainty is not due to insufficient understanding of the system; it is fundamental. Each outcome is fundamentally undetermined and even if we knew all the properties of a system's elements, events would still have more than one possible outcome in that system because each outcome occurs in a different time branch. It is true at the quantum level and it is also true at the macroscopic level as the quantum world is the framework of the latter.
See: tree shaped time
How to reconcile determinism and free will
Determinism vs Free will is a false duality as free will can be reconciled with determinism if we consider only one thing to exist as the universal absolute. The universal absolute is free from external influence because it is not part of a bigger system. The deterministic "laws" and the free will to decide are then both the same exact phenomenon.