NVIDIA PhysX is a software physics engine developed by NVIDIA. It[SQ]s used in video games and other applications to simulate realistic physical interactions, such as :

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Rigid body dynamics:

How solid objects move and collide with each other (e.g., a ball bouncing, cars crashing).
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Soft body dynamics:

How deformable objects behave (e.g., cloth, water, hair).
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Fluid dynamics:

Simulating the movement and behavior of liquids and gases.
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Particle systems:

Creating effects like smoke, fire, and explosions.

What it does for Windows:

The Windows version of NVIDIA PhysX provides the necessary runtime libraries and drivers to enable games and applications to use the PhysX engine. Without it, games and applications that rely on PhysX for their physics will either not run correctly or will have severely degraded physics simulations (or no physics at all).

Important Notes:

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Not always required:

While many games *use* PhysX, they don[SQ]t always *require* a separate installation of the PhysX runtime. Many games now bundle the necessary files within their installers.
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CPU vs. GPU acceleration:

PhysX can utilize either the CPU or the GPU for processing. GPU acceleration (using a compatible NVIDIA graphics card) is significantly faster and produces more realistic results, but CPU processing is a fallback option if a compatible GPU isn[SQ]t available.
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Generally outdated:

While still used by some games, PhysX[SQ]s development has slowed considerably, with newer game engines often using their own proprietary or other physics engines.

In short, NVIDIA PhysX for Windows is a crucial component for many games to render realistic physics, though its importance has decreased in recent years due to the rise of alternative physics engines.