Gas or smoke simulations are a subset of the fluids system, and can be used for simulating collections of airborne solids, liquid particulates and gases, such as those that make up smoke. It simulates the fluid movement of air and generates animated Voxel textures representing the density, heat, and velocity of other fluids or suspended particles (e.g. smoke) which can be used for rendering.
Gases or smoke are emitted inside of a Domain from a mesh object or particle system. The smoke movement is controlled by airflow inside the domain, which can be influenced by Effector objects. Smoke will also be affected by the scene's gravity and force fields. Airflow inside the domain can affect other physics simulations via the Fluid Flow force field.
There are Quick Liquid and Quick Smoke tools which will automatically create a domain object with a basic liquid or smoke and fire material.