Particle systems are a computer graphics technique used to simulate complex phenomena, such as fire, smoke, or rain. They consist of many small particles that represent individual elements of a larger system. Each particle has properties like position, velocity, and color, allowing for realistic movement and behavior when combined. These systems are often used in video games and animated films to create dynamic effects. By adjusting parameters like gravity, wind, and lifespan, artists can control how the particles interact and evolve over time, resulting in visually appealing and lifelike simulations of natural events.