Smoke is defined as a colloid, which is any substance consisting of particles substantially larger than atoms or ordinary molecules. However, these are too small to be visible to the unaided eye. It is a suspension of solid and or liquid particles in the air.
The particles in smoke are so fine they behave like gases. The force due to gravity is not large enough to allow the particles to settle, and their surface charge causes them to repel one another so the microscopic particles cannot coagulate into larger particles. In addition, collisions with high-speed molecules of atmospheric gases keep them "bouncing around."